Push Notifications (#66)

Fixes #35.

Things to do:
- [x] Register with local distributor
- [x] Register with homeserver
- [x] Recieve messages
- [x] Show notifications
- [ ] Notifications page (could come later)

Reviewed-on: Nexus/nexus#66

lib/pages/notifications.dart

@@ -0,0 +1,14 @@
+import "package:material_ui/material_ui.dart";
+import "package:hooks_riverpod/hooks_riverpod.dart";
+import "package:nexus/widgets/appbar.dart";
+
+class const NotificationsPage({final String? eventId, super.key})
+    extends HookConsumerWidget {
+  @override
+  Widget build(BuildContext context, WidgetRef ref) {
+    return Scaffold(
+      appBar: Appbar(title: Text("Notifications")),
+      body: Placeholder(),
+    );
+  }
+}