refactor: /simplify — 22 fixes from 3-agent code review

Critical:
- Fix MXC URI resolution: all avatars/images now resolve mxc:// to HTTP
- Sync persistence: only write changed rooms, batch message upserts
- lastActivityAt uses room.lastEvent.originServerTs, not creation time

High:
- Shared MatrixAvatar widget replaces 6 duplicate implementations
- CallScreen decodes roomId before LiveKit JWT fetch
- Decline button actually dismisses incoming call overlay
- EventTypes constants replace raw string literals
- LiveKitService uses lazy auth reads, onDispose disconnects

Medium:
- CallController is keepAlive with timer/room cleanup
- authRepository is keepAlive (used from keepAlive notifier)
- StreamController not closed in stopListening (crash fix)
- Index on messages.roomId for query performance
- 400ms debounce on user search
- Static DateFormat in MessageBubble
- Hardcoded strings replaced with AppConfig refs
- Duplicate isDirectMessage field removed from RoomModel
- E2EE profile claim corrected to Phase 3

Shared utilities:
- lib/shared/widgets/matrix_avatar.dart
- lib/shared/utils/mxc_url.dart
- lib/shared/utils/room_preview.dart
- lib/shared/utils/matrix_id.dart

rawJson column removed (unused, caused main-thread jsonEncode)
Schema migrated to v2 with roomId index.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lib/features/rooms/data/rooms_repository.dart

@@ -1,10 +1,12 @@
-// Version: 1.0.1 | Created: 2026-04-01
+// Version: 1.2.0 | Created: 2026-04-01 | Updated: 2026-04-02
 // Rooms repository. Reads room list from the Matrix SDK client.
 
 import 'package:matrix/matrix.dart';
 import 'package:riverpod_annotation/riverpod_annotation.dart';
 
 import '../../../core/network/matrix_client.dart';
+import '../../../shared/utils/mxc_url.dart';
+import '../../../shared/utils/room_preview.dart';
 import '../domain/room_model.dart';
 
 part 'rooms_repository.g.dart';
@@ -49,24 +51,11 @@ class RoomsRepository {
     return RoomModel(
       id: room.id,
       displayName: room.getLocalizedDisplayname(),
-      avatarUrl: room.avatar?.toString(),
-      lastMessagePreview: _lastMessagePreview(room),
-      lastActivityAt: room.timeCreated,
+      avatarUrl: resolveMxcUrl(_client, room.avatar),
+      lastMessagePreview: lastMessagePreview(room),
+      lastActivityAt: room.lastEvent?.originServerTs ?? room.timeCreated,
       unreadCount: room.notificationCount,
-      isDirectMessage: room.isDirectChat,
       isDirect: room.isDirectChat,
     );
   }
-
-  String? _lastMessagePreview(Room room) {
-    final lastEvent = room.lastEvent;
-    if (lastEvent == null) return null;
-
-    return switch (lastEvent.type) {
-      'm.room.message' => lastEvent.body,
-      'm.room.encrypted' => 'Encrypted message',
-      'm.sticker' => 'Sticker',
-      _ => null,
-    };
-  }
 }