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/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}