feat: web push notifications, image fullscreen, public room browser

- Web Push: push_sw.js service worker + VAPID subscription registered
  on login; Matrix pusher registered pointing to Sygnal on
  https://matrix.m8chat.au/_matrix/push/v1/notify
- Chat: tap image -> fullscreen InteractiveViewer (pinch/zoom)
- Rooms: public room browser via Matrix room directory (FAB + empty state)

Build fixes applied to web_push_service.dart:
  - dart:js_interop_unsafe import added for getProperty on JSObject
  - client.setPusher -> client.postPusher (matrix 0.33.0 API)
  - keyBytes.toJS removed (JSUint8Array already a JS type)
  - register() script URL coerced with .toJS (String -> JSAny)

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

lib/core/auth/auth_notifier.dart

@@ -1,15 +1,19 @@
-// Version: 2.0.0 | Created: 2026-04-01 | Updated: 2026-04-11
+// Version: 2.1.0 | Created: 2026-04-01 | Updated: 2026-04-27
 // Riverpod notifier that owns the auth state machine.
 //
 // WEB SECURITY MODEL: Every visit requires a fresh login. No session
 // persistence. Only the Matrix device ID is saved across sessions so
 // encryption keys accumulate on one device instead of creating ghosts.
 
+import 'dart:async';
+
 import 'package:flutter/foundation.dart';
 import 'package:riverpod_annotation/riverpod_annotation.dart';
 
 import '../../features/auth/data/auth_repository.dart';
 import '../../features/auth/domain/auth_failure.dart';
+import '../network/matrix_client.dart';
+import '../push/web_push_service.dart';
 import '../storage/sync_persistence_service.dart';
 import 'auth_state.dart';
 
@@ -51,6 +55,10 @@ class AuthNotifier extends _$AuthNotifier {
       try {
         ref.read(syncPersistenceServiceProvider).start();
       } catch (_) {}
+
+      // Register Web Push subscription (fire-and-forget, non-fatal).
+      final client = ref.read(matrixClientProvider);
+      unawaited(setupWebPush(client));
     } on AuthFailure catch (failure) {
       state = AuthState.unauthenticated(failure: failure.userMessage);
     }