fix: register PWA service worker via virtual:pwa-register for offline support

@vite-pwa/sveltekit does not auto-inject SW registration into SSR HTML.
Add explicit registration via virtual:pwa-register in root layout onMount,
inject webmanifest link via virtual:pwa-info, disable SvelteKit's built-in
SW registration to avoid conflicts, and add required dependencies.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

package.json

@@ -84,7 +84,9 @@
     "typescript": "^5.9.3",
     "vite": "^7.3.1",
     "vite-plugin-istanbul": "^7.2.1",
+    "vite-plugin-pwa": "^1.2.0",
     "vitest": "^3.2.4",
+    "workbox-window": "^7.4.0",
     "yaml": "^2.8.2"
   },
   "pnpm": {