feat(pwa): installable web app + landing web entry

Make the web SPA an installable PWA and surface it to users:

- manifest.webmanifest + an install-only service worker + 192/512/maskable
  icons (ui/public); PWA head tags in index.html; the .webmanifest MIME type
  registered in the gateway (the distroless image has no /etc/mime.types).
- Platform-adaptive install CTA (components/InstallApp.svelte + lib/pwa):
  one-tap on Chromium, manual Add-to-Home-Screen instructions on iOS Safari,
  hidden elsewhere / once installed / inside a Mini App. Shown under the
  logged-out login card and at the bottom of Settings.
- The landing gains a third entry linking /app/ (the brand tile), with a
  caption under all three (Telegram / VK / Веб-версия).

The service worker is navigation-only (network-first, cached-shell fallback);
hashed assets and the Connect stream are untouched. It exists to satisfy
Chromium's installability requirement and is the single growth point for a
future opt-in offline mode.

Tests: pwa.ts unit tests; a webui probe (manifest/sw.js content-type + the
SPA-fallback-to-HTML trap); e2e for the one-tap CTA, the iOS instructions
modal and the landing web entry. Docs: ARCHITECTURE §13, FUNCTIONAL (+ru),
gateway README.

ui/src/lib/app.svelte.ts

@@ -35,6 +35,7 @@ import {
 } from './telegram';
 import { onVKPath, insideVK, vkInit, vkClose, vkDisableSwipeBack, vkLaunchParams, vkUserName, vkStartParam, vkOnScheme, vkOnInsets, vkSetViewSettings, appearanceForBg } from './vk';
 import { pendingVKLink, type VKLinkCallback } from './vkid';
+import { registerServiceWorker } from './pwa.svelte';
 import { haptic } from './haptics';
 import { CLOUD_PREFS_KEY, decodeClientPrefs, encodeClientPrefs } from './cloudprefs';
 import { parseStartParam } from './deeplink';
@@ -783,6 +784,11 @@ export async function bootstrap(): Promise<void> {
     return;
   }
 
+  // On the plain web (both Mini-App branches returned above) register the install-only service
+  // worker so the app is installable — Chromium needs a registered SW, notably on Android. Web-only
+  // and skipped in the mock build; see lib/pwa.svelte.
+  registerServiceWorker();
+
   const saved = await loadSession();
   // A VK ID web-link callback (?code&device_id&state) rides the URL after the redirect back
   // from VK; capture and clear it here (it needs the restored session to link against).

ui/src/lib/i18n/en.ts

@@ -234,6 +234,17 @@ export const en = {
   'landing.tagline': 'Play Scrabble with friends, random opponent or an AI.',
   'landing.playTelegram': 'Play in Telegram',
   'landing.playVK': 'Play on VK',
+  'landing.playWeb': 'Play in your browser',
+  'landing.captionTelegram': 'Telegram',
+  'landing.captionVK': 'VK',
+  'landing.captionWeb': 'Web',
+
+  'install.title': 'Install the app',
+  'install.subtitle': 'Put the app icon on your desktop (home screen) to open the game in one tap.',
+  'install.iosTitle': 'Add to Home Screen',
+  'install.iosStep1': 'Tap the Share button in the browser toolbar.',
+  'install.iosStep2': 'Choose “Add to Home Screen”.',
+  'install.iosStep3': 'Tap “Add”.',
 
   'lang.en': 'English',
   'lang.ru': 'Русский',

ui/src/lib/i18n/ru.ts

@@ -234,6 +234,17 @@ export const ru: Record<MessageKey, string> = {
   'landing.tagline': 'Играй в «Эрудита» с друзьями, случайным соперником или ИИ-роботом.',
   'landing.playTelegram': 'Играть в Telegram',
   'landing.playVK': 'Играть во ВКонтакте',
+  'landing.playWeb': 'Играть в браузере',
+  'landing.captionTelegram': 'Telegram',
+  'landing.captionVK': 'VK',
+  'landing.captionWeb': 'Веб-версия',
+
+  'install.title': 'Установить приложение',
+  'install.subtitle': 'Поместите иконку приложения на рабочий стол (домашний экран), чтобы открывать игру одним нажатием.',
+  'install.iosTitle': 'На экран «Домой»',
+  'install.iosStep1': 'Нажмите кнопку «Поделиться» на панели браузера.',
+  'install.iosStep2': 'Выберите «На экран „Домой“».',
+  'install.iosStep3': 'Нажмите «Добавить».',
 
   'lang.en': 'English',
   'lang.ru': 'Русский',

ui/src/lib/pwa.svelte.ts

@@ -0,0 +1,88 @@
+// Reactive PWA install runtime: captures the Chromium install prompt, tracks the installed
+// state, and registers the install-only service worker. The pure platform branching lives in
+// pwa.ts. Structured after the other reactive lib modules (app.svelte.ts, router.svelte.ts).
+
+import { isIosSafari, isStandalone, resolveInstallMode, type InstallMode } from './pwa';
+import { insideTelegram } from './telegram';
+import { insideVK } from './vk';
+
+/** inMiniApp reports whether the app runs inside a Telegram or VK Mini App webview, where a web
+ *  install makes no sense (the platform manages its own launcher). Kept here, out of the pure
+ *  pwa.ts, so that module stays free of app imports for the node unit tests. */
+function inMiniApp(): boolean {
+  return insideTelegram() || insideVK();
+}
+
+// beforeinstallprompt is a Chromium-only event, absent from the DOM lib types.
+interface BeforeInstallPromptEvent extends Event {
+  readonly userChoice: Promise<{ outcome: 'accepted' | 'dismissed'; platform: string }>;
+  prompt(): Promise<void>;
+}
+declare global {
+  interface WindowEventMap {
+    beforeinstallprompt: BeforeInstallPromptEvent;
+    appinstalled: Event;
+  }
+}
+
+// Not named `state` (a Svelte-check hazard: `$state` would then read as a store subscription).
+const install = $state<{ deferred: BeforeInstallPromptEvent | null; installed: boolean }>({
+  deferred: null,
+  installed: isStandalone(),
+});
+
+/**
+ * installMode is the reactive install-CTA mode for the current platform and any captured prompt.
+ * Read inside a component's markup / $derived so the CTA updates when a Chromium prompt arrives
+ * (beforeinstallprompt fires shortly after load) or the app is installed.
+ */
+export function installMode(): InstallMode {
+  return resolveInstallMode({
+    deferredAvailable: install.deferred !== null,
+    standalone: install.installed,
+    iosSafari: isIosSafari(),
+    inMiniApp: inMiniApp(),
+  });
+}
+
+/**
+ * initInstallListeners wires the Chromium install prompt and the installed event. Harmless on
+ * any platform — the events fire only in an installable browser — so it may run unconditionally
+ * at startup, early enough to catch a prompt that fires before the CTA mounts.
+ */
+export function initInstallListeners(): void {
+  if (typeof window === 'undefined') return;
+  window.addEventListener('beforeinstallprompt', (e) => {
+    // Suppress Chromium's mini-infobar; the app presents its own CTA instead.
+    e.preventDefault();
+    install.deferred = e;
+  });
+  window.addEventListener('appinstalled', () => {
+    install.deferred = null;
+    install.installed = true;
+  });
+}
+
+/**
+ * promptInstall shows the native Chromium install dialog. It MUST be called directly from a user
+ * gesture (the deferred prompt is gesture-gated — no await before it). A deferred prompt is
+ * single-use, so it is cleared here; a no-op when nothing was captured.
+ */
+export async function promptInstall(): Promise<void> {
+  const deferred = install.deferred;
+  if (!deferred) return;
+  install.deferred = null;
+  await deferred.prompt();
+}
+
+/**
+ * registerServiceWorker registers the install-only worker (ui/public/sw.js). Web-only: skipped in
+ * the mock build (a real worker would perturb the Playwright run) and inside a Telegram/VK Mini
+ * App. Best-effort — a failed registration only means the app is not installable.
+ */
+export function registerServiceWorker(): void {
+  if (typeof navigator === 'undefined' || !('serviceWorker' in navigator)) return;
+  if (import.meta.env.MODE === 'mock') return;
+  if (inMiniApp()) return;
+  void navigator.serviceWorker.register('sw.js').catch(() => {});
+}

ui/src/lib/pwa.test.ts

@@ -0,0 +1,96 @@
+import { afterEach, describe, expect, it, vi } from 'vitest';
+import { isIosSafari, isStandalone, resolveInstallMode } from './pwa';
+
+describe('resolveInstallMode', () => {
+  const base = { deferredAvailable: false, standalone: false, iosSafari: false, inMiniApp: false };
+
+  it('hides inside a Mini App even with a captured prompt', () => {
+    expect(resolveInstallMode({ ...base, inMiniApp: true, deferredAvailable: true })).toBe('hidden');
+  });
+
+  it('hides once the app is installed (standalone)', () => {
+    expect(resolveInstallMode({ ...base, standalone: true, deferredAvailable: true })).toBe('hidden');
+  });
+
+  it('offers one-tap when a Chromium prompt was captured', () => {
+    expect(resolveInstallMode({ ...base, deferredAvailable: true })).toBe('oneTap');
+  });
+
+  it('offers iOS instructions on iOS Safari without a prompt', () => {
+    expect(resolveInstallMode({ ...base, iosSafari: true })).toBe('iosInstructions');
+  });
+
+  it('prefers one-tap over iOS instructions when both apply', () => {
+    expect(resolveInstallMode({ ...base, deferredAvailable: true, iosSafari: true })).toBe('oneTap');
+  });
+
+  it('hides on an unsupported browser (no prompt, not iOS)', () => {
+    expect(resolveInstallMode(base)).toBe('hidden');
+  });
+});
+
+describe('isStandalone', () => {
+  afterEach(() => vi.unstubAllGlobals());
+
+  it('is true when the display-mode media query matches', () => {
+    vi.stubGlobal('window', {
+      matchMedia: (q: string) => ({ matches: q.includes('standalone') }),
+      navigator: {},
+    });
+    expect(isStandalone()).toBe(true);
+  });
+
+  it('is true for the iOS navigator.standalone flag', () => {
+    vi.stubGlobal('window', {
+      matchMedia: () => ({ matches: false }),
+      navigator: { standalone: true },
+    });
+    expect(isStandalone()).toBe(true);
+  });
+
+  it('is false in an ordinary browser tab', () => {
+    vi.stubGlobal('window', {
+      matchMedia: () => ({ matches: false }),
+      navigator: { standalone: false },
+    });
+    expect(isStandalone()).toBe(false);
+  });
+});
+
+describe('isIosSafari', () => {
+  afterEach(() => vi.unstubAllGlobals());
+
+  const IPHONE_SAFARI =
+    'Mozilla/5.0 (iPhone; CPU iPhone OS 17_0 like Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/17.0 Mobile/15E148 Safari/604.1';
+  const IPHONE_CHROME =
+    'Mozilla/5.0 (iPhone; CPU iPhone OS 17_0 like Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) CriOS/120.0 Mobile/15E148 Safari/604.1';
+  const ANDROID_CHROME =
+    'Mozilla/5.0 (Linux; Android 13) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/120.0 Mobile Safari/537.36';
+  const IPAD_SAFARI =
+    'Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/17.0 Safari/605.1.15';
+
+  it('is true for Safari on iPhone', () => {
+    vi.stubGlobal('navigator', { userAgent: IPHONE_SAFARI, maxTouchPoints: 5 });
+    expect(isIosSafari()).toBe(true);
+  });
+
+  it('is true for Safari on iPadOS (Mac UA + touch points)', () => {
+    vi.stubGlobal('navigator', { userAgent: IPAD_SAFARI, maxTouchPoints: 5 });
+    expect(isIosSafari()).toBe(true);
+  });
+
+  it('is false for a desktop Mac (no touch points)', () => {
+    vi.stubGlobal('navigator', { userAgent: IPAD_SAFARI, maxTouchPoints: 0 });
+    expect(isIosSafari()).toBe(false);
+  });
+
+  it('is false for Chrome on iOS (CriOS)', () => {
+    vi.stubGlobal('navigator', { userAgent: IPHONE_CHROME, maxTouchPoints: 5 });
+    expect(isIosSafari()).toBe(false);
+  });
+
+  it('is false for Android Chrome', () => {
+    vi.stubGlobal('navigator', { userAgent: ANDROID_CHROME, maxTouchPoints: 5 });
+    expect(isIosSafari()).toBe(false);
+  });
+});

ui/src/lib/pwa.ts

@@ -0,0 +1,57 @@
+// Pure PWA install helpers, kept out of the Svelte layer (and free of app imports) so the
+// platform branching is unit-testable in the node test env. The reactive runtime (the captured
+// install prompt, the Mini-App gate, the service-worker registration) lives in pwa.svelte.ts.
+
+/** InstallMode is how the install call-to-action resolves for the current platform. */
+export type InstallMode = 'oneTap' | 'iosInstructions' | 'hidden';
+
+/**
+ * isStandalone reports whether the app is already running as an installed PWA — launched from
+ * the home screen / desktop rather than a browser tab. Both the standard display-mode media
+ * query and the iOS-only navigator.standalone flag are checked. Returns false with no window
+ * (e.g. under the unit tests).
+ */
+export function isStandalone(): boolean {
+  if (typeof window === 'undefined') return false;
+  const displayMode = window.matchMedia?.('(display-mode: standalone)').matches ?? false;
+  const iosStandalone =
+    (window.navigator as Navigator & { standalone?: boolean }).standalone === true;
+  return displayMode || iosStandalone;
+}
+
+/**
+ * isIosSafari reports whether the app runs in Safari on iOS / iPadOS — the only browser there
+ * that can add a site to the home screen, and one with no programmatic install (it never fires
+ * beforeinstallprompt). In-app webviews (Telegram, VK) and the iOS Chrome/Firefox/Edge wrappers
+ * are excluded, as is desktop Safari. Used to switch the CTA to manual Share -> Add-to-Home-Screen
+ * instructions.
+ */
+export function isIosSafari(): boolean {
+  if (typeof navigator === 'undefined') return false;
+  const ua = navigator.userAgent;
+  // iPadOS 13+ reports a Mac UA but exposes touch points — catch that as iOS too.
+  const isIos =
+    /iPhone|iPod|iPad/.test(ua) || (/Macintosh/.test(ua) && (navigator.maxTouchPoints ?? 0) > 1);
+  if (!isIos) return false;
+  // Safari only: the iOS Chrome (CriOS), Firefox (FxiOS), Edge (EdgiOS), Opera (OPiOS) and the
+  // Google-app (GSA) wrappers all carry "Safari" but cannot add to the home screen from our CTA.
+  return /Safari/.test(ua) && !/CriOS|FxiOS|EdgiOS|OPiOS|GSA/.test(ua);
+}
+
+/**
+ * resolveInstallMode chooses how the install CTA should behave from the platform facts:
+ * `deferredAvailable` — a beforeinstallprompt event was captured (Chromium one-tap); `standalone`
+ * — already installed; `iosSafari` — manual instructions apply; `inMiniApp` — running inside a
+ * Telegram/VK Mini App. The CTA is offered only on the plain web and never once installed.
+ */
+export function resolveInstallMode(facts: {
+  deferredAvailable: boolean;
+  standalone: boolean;
+  iosSafari: boolean;
+  inMiniApp: boolean;
+}): InstallMode {
+  if (facts.inMiniApp || facts.standalone) return 'hidden';
+  if (facts.deferredAvailable) return 'oneTap';
+  if (facts.iosSafari) return 'iosInstructions';
+  return 'hidden';
+}