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/spaces/presentation/spaces_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
 // Spaces screen — list of Matrix spaces with expandable child room lists.
 
 import 'package:flutter/material.dart';
@@ -6,6 +6,7 @@ import 'package:flutter_riverpod/flutter_riverpod.dart';
 import 'package:go_router/go_router.dart';
 import 'package:riverpod_annotation/riverpod_annotation.dart';
 
+import '../../../shared/widgets/matrix_avatar.dart';
 import '../data/spaces_repository.dart';
 import '../domain/space_model.dart';
 
@@ -93,9 +94,10 @@ class _SpaceTileState extends ConsumerState<_SpaceTile> {
       children: [
         // Space header row
         ListTile(
-          leading: _SpaceAvatar(
+          leading: MatrixAvatar(
             name: space.displayName,
             avatarUrl: space.avatarUrl,
+            radius: 22,
           ),
           title: Text(
             space.displayName,
@@ -160,7 +162,7 @@ class _ChildRoomList extends ConsumerWidget {
       children: rooms.map((room) {
         return ListTile(
           contentPadding: const EdgeInsets.only(left: 56, right: 16),
-          leading: _SpaceAvatar(
+          leading: MatrixAvatar(
             name: room.displayName,
             avatarUrl: room.avatarUrl,
             radius: 18,
@@ -180,41 +182,6 @@ class _ChildRoomList extends ConsumerWidget {
   }
 }
 
-// ---------------------------------------------------------------------------
-// Avatar widget
-// ---------------------------------------------------------------------------
-
-class _SpaceAvatar extends StatelessWidget {
-  const _SpaceAvatar({required this.name, this.avatarUrl, this.radius = 22});
-
-  final String name;
-  final String? avatarUrl;
-  final double radius;
-
-  @override
-  Widget build(BuildContext context) {
-    final initials = name.isNotEmpty ? name[0].toUpperCase() : '?';
-    if (avatarUrl != null) {
-      return CircleAvatar(
-        radius: radius,
-        backgroundImage: NetworkImage(avatarUrl!),
-      );
-    }
-    return CircleAvatar(
-      radius: radius,
-      backgroundColor: Theme.of(context).colorScheme.secondary.withAlpha(51),
-      child: Text(
-        initials,
-        style: TextStyle(
-          fontSize: radius * 0.7,
-          color: Theme.of(context).colorScheme.secondary,
-          fontWeight: FontWeight.bold,
-        ),
-      ),
-    );
-  }
-}
-
 // ---------------------------------------------------------------------------
 // Empty state
 // ---------------------------------------------------------------------------