Add PWA manifest, icons and offline service worker to the admin app

- Static web app manifest (standalone, brand-token colours) with 192/512
  "any" icons and a 512 maskable icon, generated as real PNGs by
  admin/scripts/generate-icons.mjs (zlib only, no network).
- Hand-written public/sw.js: precaches the shell on install, prunes stale
  caches on activate, falls back to the cached index.html for offline
  navigations, serves same-origin static assets cache-first; never
  handles cross-origin or non-GET requests.
- registerServiceWorker() registers /sw.js only in production builds and
  logs on failure; called from main.tsx. index.html links the manifest,
  theme colour and apple-touch-icon.
- Vitest coverage for manifest installability, icon PNG dimensions, the
  worker's install/activate/fetch behaviour and the production guard.

Task: 96ab3c0b-7554-4098-8af1-2be7815f8929

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

admin/public/manifest.webmanifest

@@ -0,0 +1,14 @@
+{
+  "name": "NestQuest Admin",
+  "short_name": "NestQuest",
+  "start_url": "/",
+  "scope": "/",
+  "display": "standalone",
+  "background_color": "#f4f4f6",
+  "theme_color": "#3b3ab8",
+  "icons": [
+    { "src": "/icons/icon-192.png", "sizes": "192x192", "type": "image/png", "purpose": "any" },
+    { "src": "/icons/icon-512.png", "sizes": "512x512", "type": "image/png", "purpose": "any" },
+    { "src": "/icons/icon-maskable-512.png", "sizes": "512x512", "type": "image/png", "purpose": "maskable" }
+  ]
+}