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/src/lib/utils/pwa.ts

@@ -0,0 +1,14 @@
+/**
+ * Unregisters all service workers and clears all caches, then reloads.
+ * Use this when the app feels stale or to force a clean re-fetch of all assets.
+ */
+export async function resetPwa(): Promise<void> {
+	if ('serviceWorker' in navigator) {
+		const registrations = await navigator.serviceWorker.getRegistrations();
+		await Promise.all(registrations.map((r) => r.unregister()));
+	}
+	if ('caches' in window) {
+		const keys = await caches.keys();
+		await Promise.all(keys.map((k) => caches.delete(k)));
+	}
+}