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/chat/presentation/chat_screen.dart

@@ -1,4 +1,4 @@
-// Version: 1.1.0 | Created: 2026-04-01
+// Version: 1.2.0 | Created: 2026-04-01 | Updated: 2026-04-02
 // Full chat screen — timeline + input + typing indicators + read receipts
 // + long-press context menu (reply, react, copy, delete).
 
@@ -10,6 +10,9 @@ import 'package:go_router/go_router.dart';
 import 'package:matrix/matrix.dart' show MatrixFile;
 
 import '../../../core/network/matrix_client.dart';
+import '../../../shared/utils/matrix_id.dart';
+import '../../../shared/utils/mxc_url.dart';
+import '../../../shared/widgets/matrix_avatar.dart';
 import '../domain/message_model.dart';
 import 'chat_controller.dart';
 import 'message_bubble.dart';
@@ -52,14 +55,14 @@ class _ChatScreenState extends ConsumerState<ChatScreen> {
     final client = ref.watch(matrixClientProvider);
     final room = client.getRoomById(_decodedRoomId);
     final roomName = room?.getLocalizedDisplayname() ?? 'Chat';
-    final roomAvatar = room?.avatar?.toString();
+    final roomAvatar = resolveMxcUrl(client, room?.avatar);
 
     return Scaffold(
       appBar: AppBar(
         titleSpacing: 0,
         title: Row(
           children: [
-            _RoomAvatarSmall(name: roomName, avatarUrl: roomAvatar),
+            MatrixAvatar(name: roomName, avatarUrl: roomAvatar, radius: 18),
             const SizedBox(width: 10),
             Flexible(child: Text(roomName, overflow: TextOverflow.ellipsis)),
           ],
@@ -105,40 +108,6 @@ class _ChatScreenState extends ConsumerState<ChatScreen> {
   }
 }
 
-// ---------------------------------------------------------------------------
-// Room avatar
-// ---------------------------------------------------------------------------
-
-class _RoomAvatarSmall extends StatelessWidget {
-  const _RoomAvatarSmall({required this.name, this.avatarUrl});
-
-  final String name;
-  final String? avatarUrl;
-
-  @override
-  Widget build(BuildContext context) {
-    final initials = name.isNotEmpty ? name[0].toUpperCase() : '?';
-    if (avatarUrl != null) {
-      return CircleAvatar(
-        radius: 18,
-        backgroundImage: NetworkImage(avatarUrl!),
-      );
-    }
-    return CircleAvatar(
-      radius: 18,
-      backgroundColor: Theme.of(context).colorScheme.primary.withAlpha(51),
-      child: Text(
-        initials,
-        style: TextStyle(
-          fontSize: 14,
-          fontWeight: FontWeight.bold,
-          color: Theme.of(context).colorScheme.primary,
-        ),
-      ),
-    );
-  }
-}
-
 // ---------------------------------------------------------------------------
 // Timeline
 // ---------------------------------------------------------------------------
@@ -337,7 +306,7 @@ class _TypingIndicator extends ConsumerWidget {
     // typingUsers returns Users currently typing (excluding self).
     final typing = room.typingUsers
         .where((u) => u.id != client.userID)
-        .map((u) => u.displayName ?? u.id.split(':').first)
+        .map((u) => u.displayName ?? u.id.matrixLocalpart)
         .toList();
 
     if (typing.isEmpty) return const SizedBox(height: 4);