feat(ui): drop Telegram fullscreen; own back chevron everywhere; hidden debug panel

Finalises the Telegram Mini App navigation work after on-device testing (Pixel
10 / Android 17 + iOS, fresh beta clients):

- Remove requestFullscreen entirely. Immersive fullscreen hid Telegram's native
  header (and its BackButton) and the Android system swipe-back minimised the
  app; the owner prefers the windowed full-size (expand) presentation, so the
  app never requests fullscreen on any platform now.
- The app's own back chevron (Header, showBack = !!back) drives back-navigation
  on every platform; the native Telegram BackButton is dropped — it does not
  render in the windowed Mini App (backVisible=false on iOS and Android), so
  relying on it lost back navigation (notably none on iOS).
- Replace the temporary always-on diagnostic overlay with a hidden debug panel
  (components/DebugPanel): ten quick taps on the header title open it; it shows a
  privacy-safe client diagnostic snapshot (app version, locale, online, userId,
  Telegram chrome / viewport / SDK state — no secrets, no IP) and shares it via
  the OS share sheet / clipboard; a tap anywhere except Share dismisses it.
- Drop the now-dead telegramRequestFullscreen / telegramBackButton /
  isTelegramAndroid helpers and the iOS-fullscreen unit test.

Telegram has no native non-modal notification API (only modal showPopup /
showAlert), so in-app toasts stay ours. Docs: UI_DESIGN.md.

ui/src/App.svelte

@@ -4,11 +4,11 @@
   import { app, bootstrap } from './lib/app.svelte';
   import { router, type RouteName } from './lib/router.svelte';
   import { t } from './lib/i18n/index.svelte';
-  import { insideTelegram, telegramChromeDiag } from './lib/telegram';
   import Toast from './components/Toast.svelte';
   import Splash from './components/Splash.svelte';
   import StaleInviteModal from './components/StaleInviteModal.svelte';
   import WelcomeRedeemModal from './components/WelcomeRedeemModal.svelte';
+  import DebugPanel from './components/DebugPanel.svelte';
   import Login from './screens/Login.svelte';
   import Lobby from './screens/Lobby.svelte';
   import NewGame from './screens/NewGame.svelte';
@@ -125,12 +125,8 @@
   <Splash />
 {/if}
 
-<!-- TEMP DIAGNOSTIC overlay (Android nav debug) — REMOVE before merge -->
-{#if app.ready && insideTelegram()}
-  {#key router.route.name + (router.route.params.id ?? '')}
-    <pre
-      style="position:fixed;left:0;right:0;bottom:0;z-index:9999;margin:0;padding:6px 8px;font:10px/1.35 ui-monospace,monospace;white-space:pre-wrap;overflow-wrap:anywhere;background:rgba(0,0,0,0.85);color:#3f3;max-height:45vh;overflow:auto;pointer-events:none">{telegramChromeDiag()}</pre>
-  {/key}
+{#if app.debugOpen}
+  <DebugPanel />
 {/if}
 
 <style>

ui/src/components/DebugPanel.svelte

@@ -0,0 +1,71 @@
+<script lang="ts">
+  // Hidden on-device debug panel, opened by tapping the header title ten times (Header.svelte) and
+  // closed by tapping anywhere except the Share control. It shows a privacy-safe client diagnostic
+  // snapshot (no secrets, no initData values, no IP) and shares it through the OS share sheet (or a
+  // clipboard copy on desktop) — a support aid for reproducing client-specific issues, e.g. the
+  // Telegram Android presentation quirks. Drawn from the top, just under the app header.
+  import { app, closeDebug } from '../lib/app.svelte';
+  import { connection } from '../lib/connection.svelte';
+  import { shareText } from '../lib/share';
+  import { telegramChromeDiag } from '../lib/telegram';
+
+  const report = [
+    `app: ${__APP_VERSION__}`,
+    `locale: ${app.locale}  theme: ${app.theme}  reduceMotion: ${app.reduceMotion}`,
+    `online: ${connection.online}  streamAlive: ${app.streamAlive}`,
+    `userId: ${app.session?.userId ?? '—'}  guest: ${app.profile?.isGuest ?? '—'}`,
+    telegramChromeDiag(),
+  ].join('\n');
+
+  let label = $state('Share');
+  async function share(e: MouseEvent): Promise<void> {
+    e.stopPropagation(); // a tap on Share shares; it must not also close the panel
+    const r = await shareText(report, `Scrabble debug ${__APP_VERSION__}`);
+    if (r === 'copied') {
+      label = 'Copied';
+      setTimeout(() => (label = 'Share'), 1500);
+    }
+  }
+</script>
+
+<!-- svelte-ignore a11y_click_events_have_key_events -->
+<!-- svelte-ignore a11y_no_static_element_interactions -->
+<div class="overlay" onclick={closeDebug}>
+  <button class="share" onclick={share}>{label}</button>
+  <pre class="body">{report}</pre>
+</div>
+
+<style>
+  .overlay {
+    position: fixed;
+    inset: 0;
+    z-index: 10000;
+    /* Drawn from the top; the content clears the app header (~56px + the device safe-area). */
+    padding: calc(var(--tg-safe-top, 0px) + 56px) 12px 16px;
+    background: rgba(0, 0, 0, 0.82);
+    overflow: auto;
+    display: flex;
+    flex-direction: column;
+    gap: 10px;
+    align-items: flex-start;
+  }
+  .share {
+    flex: 0 0 auto;
+    padding: 7px 16px;
+    border: 1px solid var(--accent);
+    background: var(--accent);
+    color: var(--accent-text);
+    border-radius: var(--radius-sm);
+    font-size: 0.95rem;
+  }
+  .body {
+    margin: 0;
+    width: 100%;
+    white-space: pre-wrap;
+    overflow-wrap: anywhere;
+    font-family: ui-monospace, SFMono-Regular, Menlo, monospace;
+    font-size: 11px;
+    line-height: 1.45;
+    color: #d6e6ff;
+  }
+</style>

ui/src/components/Header.svelte

@@ -2,7 +2,7 @@
   import { navigate } from '../lib/router.svelte';
   import { connection } from '../lib/connection.svelte';
   import { t } from '../lib/i18n/index.svelte';
-  import { app } from '../lib/app.svelte';
+  import { app, openDebug } from '../lib/app.svelte';
   import Spinner from './Spinner.svelte';
   import AdBanner from './AdBanner.svelte';
 
@@ -12,6 +12,19 @@
   // and out of Telegram. The native Telegram BackButton is not used: it does not render reliably in
   // the windowed Mini App (relying on it would lose back navigation there).
   const showBack = $derived(!!back);
+
+  // Ten quick taps on the title open the hidden debug panel (components/DebugPanel) — a support aid.
+  let titleTaps = 0;
+  let lastTitleTap = 0;
+  function onTitleTap(): void {
+    const now = Date.now();
+    titleTaps = now - lastTitleTap < 400 ? titleTaps + 1 : 1;
+    lastTitleTap = now;
+    if (titleTaps >= 10) {
+      titleTaps = 0;
+      openDebug();
+    }
+  }
 </script>
 
 <header class="nav" class:grow>
@@ -24,7 +37,9 @@
       <span class="spacer"></span>
     {/if}
     {#if connection.online}
-      <h1>{title}</h1>
+      <!-- svelte-ignore a11y_click_events_have_key_events -->
+      <!-- svelte-ignore a11y_no_noninteractive_element_interactions -->
+      <h1 onclick={onTitleTap}>{title}</h1>
     {:else}
       <h1 class="connecting"><Spinner /> <span>{t('connection.connecting')}</span></h1>
     {/if}

ui/src/lib/app.svelte.ts

@@ -25,7 +25,6 @@ import {
   telegramLaunch,
   type TelegramLaunch,
   telegramOnEvent,
-  telegramRequestFullscreen,
   telegramSetChrome,
 } from './telegram';
 import { parseStartParam } from './deeplink';
@@ -56,6 +55,9 @@ export const app = $state<{
    *  launch-error screen (screens/TelegramLaunchError) — a shareable probe for why Telegram
    *  delivered no initData (seen on some Android clients) — instead of bouncing to the landing. */
   launchError: TelegramDiag | null;
+  /** Whether the hidden on-device debug panel (components/DebugPanel) is open — toggled by tapping
+   *  the header title ten times in quick succession. A support aid; carries no secrets. */
+  debugOpen: boolean;
   /** Whether the lobby's first cold load has settled (success or error). The loading splash
    *  (components/Splash.svelte) watches it to know when to dismiss; set by screens/Lobby. */
   lobbyReady: boolean;
@@ -106,6 +108,7 @@ export const app = $state<{
   ready: false,
   bootError: false,
   launchError: null,
+  debugOpen: false,
   lobbyReady: false,
   splashDone: false,
   streamAlive: false,
@@ -197,6 +200,16 @@ export function dismissWelcomeRedeem(): void {
   app.welcomeRedeem = false;
 }
 
+/** openDebug / closeDebug toggle the hidden on-device debug panel (components/DebugPanel), opened
+ *  by tapping the header title ten times — a support aid that shows and shares client diagnostics. */
+export function openDebug(): void {
+  app.debugOpen = true;
+}
+
+export function closeDebug(): void {
+  app.debugOpen = false;
+}
+
 /**
  * seedChatUnread sets a game's unread flags from an authoritative per-viewer REST view (the lobby
  * list, a game's state, or a move result): unread is any unread entry, message whether one of them
@@ -557,10 +570,6 @@ function applyTelegramChrome(launch: TelegramLaunch): void {
   syncTelegramChrome();
   syncTelegramSafeArea();
   telegramDisableVerticalSwipes();
-  // On mobile, go immersive fullscreen like Telegram's own Mini Apps; the fullscreenChanged
-  // listener (registered at bootstrap) then re-syncs the safe-area insets. Desktop keeps the bot's
-  // full-size window. No-op on clients predating Bot API 8.0.
-  telegramRequestFullscreen();
 }
 
 /** How long to wait for the dynamically loaded Telegram Mini App SDK before giving up and showing

ui/src/lib/telegram.test.ts

@@ -7,7 +7,6 @@ import {
   routeExternalLinkInTelegram,
   telegramLaunch,
   telegramOpenExternalLink,
-  telegramRequestFullscreen,
 } from './telegram';
 
 function stubWebApp(initData: string, startParam?: string) {
@@ -47,44 +46,6 @@ describe('telegram launch detection', () => {
   });
 });
 
-// stubClient stands up a fake WebApp on the given platform with a requestFullscreen spy, so the
-// platform gate can be asserted without a real Telegram client.
-function stubClient(platform?: string) {
-  const requestFullscreen = vi.fn();
-  vi.stubGlobal('window', { Telegram: { WebApp: { platform, requestFullscreen } } });
-  return { requestFullscreen };
-}
-
-const desktopPlatforms = ['tdesktop', 'macos', 'web', undefined];
-
-describe('telegramRequestFullscreen', () => {
-  afterEach(() => vi.unstubAllGlobals());
-
-  // TEMP: fullscreen is disabled on all platforms for an owner test (telegramRequestFullscreen is a
-  // no-op); restore this (toHaveBeenCalledOnce) and un-skip when the iOS path is restored.
-  it.skip('goes immersive fullscreen on iOS', () => {
-    const { requestFullscreen } = stubClient('ios');
-    telegramRequestFullscreen();
-    expect(requestFullscreen).toHaveBeenCalledOnce();
-  });
-
-  it('stays windowed on Android, where fullscreen hides the native back button', () => {
-    for (const p of ['android', 'android_x']) {
-      const { requestFullscreen } = stubClient(p);
-      telegramRequestFullscreen();
-      expect(requestFullscreen, `platform=${p}`).not.toHaveBeenCalled();
-    }
-  });
-
-  it('leaves desktop clients as a standard window', () => {
-    for (const p of desktopPlatforms) {
-      const { requestFullscreen } = stubClient(p);
-      telegramRequestFullscreen();
-      expect(requestFullscreen, `platform=${p}`).not.toHaveBeenCalled();
-    }
-  });
-});
-
 describe('telegramOpenExternalLink', () => {
   afterEach(() => vi.unstubAllGlobals());
 

ui/src/lib/telegram.ts

@@ -22,7 +22,6 @@ interface TelegramWebApp {
   contentSafeAreaInset?: { top: number; bottom: number; left: number; right: number };
   ready?: () => void;
   expand?: () => void;
-  requestFullscreen?: () => void;
   openTelegramLink?: (url: string) => void;
   openLink?: (url: string) => void;
   onEvent?: (event: string, handler: () => void) => void;
@@ -295,54 +294,6 @@ export function telegramHaptic(kind: Haptic): void {
   else h.impactOccurred?.(kind);
 }
 
-/**
- * telegramRequestFullscreen asks Telegram to open the Mini App in immersive fullscreen (Bot API
- * 8.0+), but only on iOS. On Android, fullscreen replaces the native header — and its BackButton —
- * with a bare close/menu pill: the back control disappears and the Android system swipe-back falls
- * through to minimising the app instead of navigating, so the app stays windowed there to keep the
- * native header + BackButton (which also captures the system back). A no-op outside Telegram, on
- * Android, on desktop, or on clients predating the method.
- */
-export function telegramRequestFullscreen(): void {
-  // TEMP (owner test): fullscreen fully disabled, incl. iOS, to confirm the Android "fullscreen
-  // look" is Telegram's own Mini App presentation, not our requestFullscreen (isFullscreen is
-  // already false on Android). Restore the iOS path after the test:
-  // if (webApp()?.platform === 'ios') webApp()?.requestFullscreen?.();
-}
-
-/** isTelegramAndroid reports whether the Mini App runs on a Telegram Android client (reported as
- *  'android', or 'android_x' for Telegram X). The native header BackButton does not render in the
- *  Android (non-fullscreen) presentation, so back navigation there uses the app's own chevron. */
-export function isTelegramAndroid(): boolean {
-  const p = webApp()?.platform;
-  return p === 'android' || p === 'android_x';
-}
-
-let backHandler: (() => void) | null = null;
-let lastBackShow = false; // TEMP diag: the last telegramBackButton(show) request
-
-/**
- * telegramBackButton shows or hides Telegram's native header back button, wiring its
- * click to onClick (replacing any previous handler). The app hides its own back chevron
- * inside Telegram so only the native control shows.
- */
-export function telegramBackButton(show: boolean, onClick?: () => void): void {
-  lastBackShow = show;
-  const b = webApp()?.BackButton;
-  if (!b) return;
-  if (backHandler) b.offClick?.(backHandler);
-  backHandler = null;
-  if (show) {
-    if (onClick) {
-      backHandler = onClick;
-      b.onClick?.(onClick);
-    }
-    b.show?.();
-  } else {
-    b.hide?.();
-  }
-}
-
 /**
  * startParamFromURL reads a startapp parameter from the page URL — a bot web_app
  * launch button carries the deep-link there rather than in initDataUnsafe.
@@ -488,9 +439,10 @@ export function collectTelegramDiag(): TelegramDiag {
 }
 
 /**
- * telegramChromeDiag is a TEMPORARY on-device readout of Telegram's viewport / fullscreen state,
- * rendered in the lobby to debug the Android fullscreen-persistence issue (the app still opens
- * fullscreen on Android even though requestFullscreen is now iOS-only). REMOVE before merge.
+ * telegramChromeDiag returns a compact, privacy-safe readout of Telegram's viewport / chrome state
+ * (platform, version, fullscreen/expanded, viewport geometry, safe-area insets, SDK-load outcome,
+ * back-button state, UA). It feeds the hidden debug panel (components/DebugPanel), opened by tapping
+ * the header title ten times. No secrets: no initData values, no IP.
  */
 export function telegramChromeDiag(): string {
   const w = webApp();
@@ -505,7 +457,7 @@ export function telegramChromeDiag(): string {
   return [
     `platform: ${w.platform ?? '—'}  version: ${w.version ?? '—'}  scheme: ${w.colorScheme ?? '—'}`,
     `isFullscreen: ${w.isFullscreen}  isExpanded: ${w.isExpanded}`,
-    `inTG: ${insideTelegram()}  backReq: ${lastBackShow}  backPresent: ${!!w.BackButton}  backVisible: ${w.BackButton?.isVisible}`,
+    `inTG: ${insideTelegram()}  sdkLoad: ${sdkLoadOutcome}  backPresent: ${!!w.BackButton}  backVisible: ${w.BackButton?.isVisible}`,
     `innerH: ${n(win?.innerHeight)}  outerH: ${n(win?.outerHeight)}  screenH: ${n(scr?.height)}  availH: ${n(scr?.availHeight)}`,
     `screenY: ${n(win?.screenY)}  vv.offTop: ${n(vv?.offsetTop)}  vv.h: ${n(vv?.height)}`,
     `tgViewportH: ${n(w.viewportHeight)}  stableH: ${n(w.viewportStableHeight)}`,