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/presentation/room_tile.dart

@@ -1,10 +1,10 @@
-// Version: 1.0.0 | Created: 2026-04-01
+// Version: 1.1.0 | Created: 2026-04-01 | Updated: 2026-04-02
 // Individual room list tile. Kept under 100 lines.
 
-import 'package:cached_network_image/cached_network_image.dart';
 import 'package:flutter/material.dart';
 import 'package:timeago/timeago.dart' as timeago;
 
+import '../../../shared/widgets/matrix_avatar.dart';
 import '../domain/room_model.dart';
 
 class RoomTile extends StatelessWidget {
@@ -19,7 +19,11 @@ class RoomTile extends StatelessWidget {
     final hasUnread = room.unreadCount > 0;
 
     return ListTile(
-      leading: _RoomAvatar(room: room),
+      leading: MatrixAvatar(
+        name: room.displayName,
+        avatarUrl: room.avatarUrl,
+        radius: 24,
+      ),
       title: Text(
         room.displayName,
         style: theme.textTheme.bodyLarge?.copyWith(
@@ -62,40 +66,6 @@ class RoomTile extends StatelessWidget {
   }
 }
 
-class _RoomAvatar extends StatelessWidget {
-  const _RoomAvatar({required this.room});
-
-  final RoomModel room;
-
-  @override
-  Widget build(BuildContext context) {
-    final theme = Theme.of(context);
-    final initials = room.displayName.isNotEmpty
-        ? room.displayName[0].toUpperCase()
-        : '?';
-
-    if (room.avatarUrl != null) {
-      return CircleAvatar(
-        radius: 24,
-        backgroundImage: CachedNetworkImageProvider(room.avatarUrl!),
-        backgroundColor: theme.colorScheme.surfaceContainerHighest,
-      );
-    }
-
-    return CircleAvatar(
-      radius: 24,
-      backgroundColor: theme.colorScheme.primary.withAlpha(51),
-      child: Text(
-        initials,
-        style: TextStyle(
-          color: theme.colorScheme.primary,
-          fontWeight: FontWeight.bold,
-        ),
-      ),
-    );
-  }
-}
-
 class _UnreadBadge extends StatelessWidget {
   const _UnreadBadge({required this.count});