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).

gateway/README.md

@@ -8,9 +8,10 @@ backend over REST/JSON, and bridges the backend's gRPC push stream to each
 client's in-app live channel. It **embeds the static UI build** (`go:embed`, baked
 in by the gateway image's node stage) and serves a **landing page** at `/` and the game
 **SPA** at `/app/` (web), `/telegram/` (the Telegram Mini App) and `/vk/` (the VK Mini App) — the single-origin model.
-The web SPA is an installable **PWA**: it serves `manifest.webmanifest` (with the `.webmanifest`
-MIME type registered in-process — the distroless image has no `/etc/mime.types`) and the
-install-only `sw.js`, both shipped unhashed from `ui/public/`.
+The web SPA is an installable **PWA**: it serves `manifest.webmanifest` (unhashed from `ui/public/`,
+with the `.webmanifest` MIME type registered in-process — the distroless image has no
+`/etc/mime.types`) and the app-shell `sw.js` (built from `ui/src/sw.ts` by vite-plugin-pwa, which
+precaches the shell + assets so an installed PWA cold-launches offline).
 Hash-named `/assets/*` are served `immutable`; the HTML shells are `no-cache`. It can also serve the
 backend's admin console at `/_gm` behind HTTP Basic-Auth for a local non-caddy run;
 in the deployed contour the front caddy owns `/_gm` (see