feat: Add push notification support with VAPID keys

- Backend: web-push integration with VAPID keys
- Push subscription endpoints (GET /api/push/public-key, POST /api/push/subscribe)
- In-memory subscription store
- Frontend: Auto-subscribe to push after notification permission granted
- Service Worker: Push event handler
- Send push notifications when torrents complete
- Works even when browser is closed

frontend/Cargo.toml

@@ -41,7 +41,13 @@ web-sys = { version = "0.3", features = [
     "Navigator",
     "Notification",
     "NotificationOptions",
-    "NotificationPermission"
+    "NotificationPermission",
+    "ServiceWorkerContainer",
+    "ServiceWorkerRegistration",
+    "PushManager",
+    "PushSubscription",
+    "PushSubscriptionOptions",
+    "PushSubscriptionOptionsInit"
 ] }
 shared = { path = "../shared" }
 tailwind_fuse = "0.3.2"