feat(offline): app-shell precache service worker (vite-plugin-pwa)

Migrate the former install-only public/sw.js to a custom (injectManifest)
service worker built from ui/src/sw.ts by vite-plugin-pwa. It precaches the
app shell + hashed assets (Workbox) so an installed web PWA cold-launches with
no network, and falls in-scope navigations back to the precached shell (the
hash router resolves the route client-side). This is the C1 prerequisite for a
usable offline mode: without it a cold offline launch cannot load the bundle.

- ui/src/sw.ts: skipWaiting + clientsClaim + cleanupOutdatedCaches +
  precacheAndRoute(__WB_MANIFEST) + a NavigationRoute fallback to index.html,
  deny-listing the RPC path and /_gm. The Connect stream and API POSTs are
  never precached nor intercepted.
- vite.config.ts: VitePWA(injectManifest); manifest:false (we ship our own),
  injectRegister:false (registration stays manual + web-only in pwa.svelte.ts),
  disabled in the mock build (Playwright unperturbed); landing + polyfills
  excluded from the precache.
- Removed public/sw.js; the registration path (dist/sw.js at /app/) is unchanged.
- New dev deps: vite-plugin-pwa + workbox-{core,precaching,routing}.

Docs: ARCHITECTURE + gateway/README. Offline cold-launch is contour-verified
(the mock e2e harness disables the SW).

ui/public/sw.js

@@ -1,53 +0,0 @@
-// Install-only service worker.
-//
-// Its sole job today is to satisfy Chromium's PWA installability requirement (a registered
-// service worker with a fetch handler) so the web app can be installed to the home screen /
-// desktop — notably on Android, where the manifest alone is not enough. It deliberately does
-// NOT cache assets or the Connect-RPC stream: only top-level navigations are handled,
-// network-first with a cached-shell fallback, so the live app, its immutable hashed assets and
-// the live event stream are never served stale.
-//
-// This is the single designated growth point for the planned offline mode (a Settings toggle,
-// default online — see docs/ARCHITECTURE.md). Extend the fetch router here; keep the same
-// /app/ scope; gate any real caching behind the toggle and coordinate updates with the boot
-// version guard.
-const SHELL = 'scrabble-shell-v1'; // bump only when the SW strategy itself changes.
-
-self.addEventListener('install', () => {
-  // No cached content to migrate carefully, so activate the new worker immediately.
-  self.skipWaiting();
-});
-
-self.addEventListener('activate', (event) => {
-  event.waitUntil(
-    (async () => {
-      const keys = await caches.keys();
-      await Promise.all(keys.filter((k) => k !== SHELL).map((k) => caches.delete(k)));
-      await self.clients.claim();
-    })(),
-  );
-});
-
-self.addEventListener('fetch', (event) => {
-  const req = event.request;
-  // Only top-level navigations are handled. Hashed assets and the Connect-RPC requests are
-  // left untouched (no respondWith -> the browser performs its default fetch), so the worker
-  // can never break the live stream or serve a stale immutable asset.
-  if (req.mode !== 'navigate') return;
-  event.respondWith(
-    (async () => {
-      try {
-        const res = await fetch(req);
-        const cache = await caches.open(SHELL);
-        // One shell entry regardless of the deep-link path: the hash router resolves the route
-        // client-side, so any offline navigation can be served the same cached shell.
-        cache.put('shell', res.clone());
-        return res;
-      } catch {
-        const cache = await caches.open(SHELL);
-        const cached = await cache.match('shell');
-        return cached ?? Response.error();
-      }
-    })(),
-  );
-});