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/message_bubble.dart

@@ -1,10 +1,11 @@
-// Version: 1.0.0 | Created: 2026-04-01
+// Version: 1.1.0 | Created: 2026-04-01 | Updated: 2026-04-02
 // Message bubble widget. Handles text, images, files, redacted, replies.
 
 import 'package:cached_network_image/cached_network_image.dart';
 import 'package:flutter/material.dart';
 import 'package:intl/intl.dart';
 
+import '../../../shared/widgets/matrix_avatar.dart';
 import '../domain/message_model.dart';
 
 class MessageBubble extends StatelessWidget {
@@ -31,7 +32,11 @@ class MessageBubble extends StatelessWidget {
             : MainAxisAlignment.start,
         children: [
           if (!isMine) ...[
-            _SenderAvatar(message: message),
+            MatrixAvatar(
+              name: message.senderDisplayName,
+              avatarUrl: message.senderAvatarUrl,
+              radius: 16,
+            ),
             const SizedBox(width: 8),
           ],
           Flexible(
@@ -63,39 +68,6 @@ class MessageBubble extends StatelessWidget {
   }
 }
 
-class _SenderAvatar extends StatelessWidget {
-  const _SenderAvatar({required this.message});
-
-  final MessageModel message;
-
-  @override
-  Widget build(BuildContext context) {
-    final initials = message.senderDisplayName.isNotEmpty
-        ? message.senderDisplayName[0].toUpperCase()
-        : '?';
-
-    if (message.senderAvatarUrl != null) {
-      return CircleAvatar(
-        radius: 16,
-        backgroundImage: CachedNetworkImageProvider(message.senderAvatarUrl!),
-      );
-    }
-
-    return CircleAvatar(
-      radius: 16,
-      backgroundColor: Theme.of(context).colorScheme.primary.withAlpha(51),
-      child: Text(
-        initials,
-        style: TextStyle(
-          fontSize: 12,
-          color: Theme.of(context).colorScheme.primary,
-          fontWeight: FontWeight.bold,
-        ),
-      ),
-    );
-  }
-}
-
 class _BubbleContent extends StatelessWidget {
   const _BubbleContent({required this.message, required this.isMine});
 
@@ -238,13 +210,15 @@ class _Timestamp extends StatelessWidget {
     required this.textColour,
   });
 
+  static final DateFormat _timeFormat = DateFormat('HH:mm');
+
   final DateTime timestamp;
   final bool isEdited;
   final Color textColour;
 
   @override
   Widget build(BuildContext context) {
-    final formatted = DateFormat('HH:mm').format(timestamp.toLocal());
+    final formatted = _timeFormat.format(timestamp.toLocal());
     final label = isEdited ? '$formatted  (edited)' : formatted;
 
     return Text(label, style: TextStyle(fontSize: 10, color: textColour));