feat(frontend): add PWA support (service worker, manifest, pwa util)

- src/service-worker.ts: cache-first app shell (build + static assets),
  network-only for /api/, offline fallback to SPA shell
- static/manifest.webmanifest: name/short_name Tanabata, theme #312F45,
  standalone display, start_url /files, icon paths for 192/512/maskable
- src/lib/utils/pwa.ts: resetPwa() — unregisters SW + clears all caches
- app.html: link manifest, theme-color meta, Apple PWA metas
- settings page: refactored to use resetPwa() from utils

Note: add /static/images/icon-192.png, icon-512.png, icon-maskable-512.png
for full installability.

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

frontend/static/manifest.webmanifest

@@ -0,0 +1,31 @@
+{
+  "name": "Tanabata",
+  "short_name": "Tanabata",
+  "description": "Multi-user tag-based file manager",
+  "start_url": "/files",
+  "scope": "/",
+  "display": "standalone",
+  "orientation": "portrait-primary",
+  "background_color": "#312F45",
+  "theme_color": "#312F45",
+  "icons": [
+    {
+      "src": "/images/icon-192.png",
+      "sizes": "192x192",
+      "type": "image/png",
+      "purpose": "any"
+    },
+    {
+      "src": "/images/icon-512.png",
+      "sizes": "512x512",
+      "type": "image/png",
+      "purpose": "any"
+    },
+    {
+      "src": "/images/icon-maskable-512.png",
+      "sizes": "512x512",
+      "type": "image/png",
+      "purpose": "maskable"
+    }
+  ]
+}