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).

docs/ARCHITECTURE.md

@@ -1238,12 +1238,15 @@ separate site; the SPA shell is `noindex`, and the landing always boots in Russi
 browser-language detection — crawlers render with arbitrary languages). The favicon set,
 `og-image.png` and `robots.txt` ship unhashed from `ui/public/` (generated by
 `assets/icons/`, same tile design as the VK loader). On the web (`/app/`) the SPA is an installable **PWA**:
-`manifest.webmanifest` and an install-only **service worker** (`sw.js`) ship unhashed from
-`ui/public/`, and the client registers the worker **web-only** — never inside a Mini App or the
-mock build. The worker intercepts only top-level navigations (network-first with a cached-shell
-fallback), leaving `/assets/*` and the Connect stream untouched; it exists to satisfy Chromium's
-installability requirement (a registered SW, needed for install on Android) and is the single
-growth point for the opt-in **offline mode** (in progress): a deliberate, device-scoped Settings
+`manifest.webmanifest` ships unhashed from `ui/public/`, while the **service worker** (`sw.js`) is
+built from `ui/src/sw.ts` by **vite-plugin-pwa** (injectManifest strategy) — the client registers it
+**web-only**, never inside a Mini App or the mock build (the plugin is disabled there entirely). It
+**precaches the app shell and the hashed assets** (Workbox) so an installed PWA cold-launches with
+no network, and falls every in-scope navigation back to the precached shell (the hash router
+resolves the route client-side); the landing page and the conditional polyfill bundle are excluded,
+and the Connect stream and runtime API POSTs are never precached nor intercepted, so the live app is
+never served stale. This satisfies Chromium's installability requirement (a registered SW, needed
+for install on Android) and powers the opt-in **offline mode** (in progress): a deliberate, device-scoped Settings
 toggle — distinct from the transient gateway-reachability signal — that tints the header blue with
 an *Offline* chip and confines play to on-device `vs_ai` games. To have data ready before the
 switch, the **Profile advertises the current dictionary version per variant** (`dict_versions`,