feat: Add PWA support with manifest and ServiceWorker integration

- Mounted static files directory to serve PWA assets like manifest.json and ServiceWorker scripts.
- Updated `auth.html` and `keys_status.html` templates:
  - Added `<link>` for manifest and icons to support Progressive Web App (PWA) features.
  - Added meta tags for theme color and Apple web app capabilities.
  - Integrated ServiceWorker registration script for offline capabilities.

app/static/manifest.json

@@ -0,0 +1,17 @@
+{
+  "name": "Gemini Balance",
+  "short_name": "GBalance",
+  "description": "Gemini API密钥管理工具",
+  "start_url": "/",
+  "display": "standalone",
+  "background_color": "#667eea",
+  "theme_color": "#764ba2",
+  "icons": [
+    {
+      "src": "/static/icons/icon-192x192.png",
+      "sizes": "192x192",
+      "type": "image/png",
+      "purpose": "any maskable"
+    }
+  ]
+}