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/vite.config.ts

@@ -2,6 +2,7 @@ import { readFileSync } from 'node:fs';
 import { resolve } from 'node:path';
 import { defineConfig, type Plugin } from 'vite';
 import { svelte } from '@sveltejs/vite-plugin-svelte';
+import { VitePWA } from 'vite-plugin-pwa';
 
 /**
  * injectBootVersion stamps the app version into index.html's boot-capability guard, replacing its
@@ -56,7 +57,32 @@ export default defineConfig(({ mode }) => ({
   },
   // emitPolyfills ships dist/polyfills.js (loaded only by old engines; see its docstring + the
   // index.html boot guard). injectBootVersion stamps the app version into that guard's diagnostic.
-  plugins: [svelte(), emitPolyfills(), injectBootVersion()],
+  plugins: [
+    svelte(),
+    emitPolyfills(),
+    injectBootVersion(),
+    // App-shell precache for the offline mode: a custom (injectManifest) service worker precaches
+    // index.html + the hashed assets so the installed web PWA cold-launches with no network. It
+    // supersedes the former install-only public/sw.js and outputs to dist/sw.js (same path, so the
+    // existing /app/ registration is unchanged). Registration is manual + web-only (injectRegister
+    // off; see pwa.svelte.ts) and the whole plugin is disabled in the mock build so Playwright is
+    // never perturbed. manifest:false — we already ship public/manifest.webmanifest.
+    VitePWA({
+      strategies: 'injectManifest',
+      srcDir: 'src',
+      filename: 'sw.ts',
+      injectRegister: false,
+      manifest: false,
+      disable: mode === 'mock',
+      injectManifest: {
+        // The app shell only: exclude the separate landing page and the conditional polyfill bundle
+        // (loaded document.write-only on an old engine; never part of the offline shell).
+        globPatterns: ['**/*.{js,css,html,svg,png,ico,woff,woff2}'],
+        globIgnores: ['**/landing*', '**/polyfills.js'],
+      },
+      devOptions: { enabled: false },
+    }),
+  ],
   server: {
     port: 5173,
     proxy: