feat: PWA support — installable on all platforms

- Add vite-plugin-pwa with autoUpdate service worker
- Generate SVG + PNG icons (192, 512, apple-touch-icon, favicon)
- Web app manifest with standalone display mode
- Workbox caching: precache assets, cache Google Fonts, NetworkFirst for API
- Apple mobile web app meta tags for iOS home screen

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

frontend/package.json

@@ -18,6 +18,7 @@
     "tailwindcss": "^4.2.1",
     "typescript": "^5.7.3",
     "vite": "^7.3.1",
+    "vite-plugin-pwa": "^1.2.0",
     "vue-tsc": "^2.2.8"
   }
 }