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/shared/utils/room_preview.dart

@@ -0,0 +1,19 @@
+// Version: 1.0.0 | Created: 2026-04-02
+// Shared utility for generating a last-message preview string from a Room.
+// Used by both RoomsRepository and SyncPersistenceService to avoid duplication.
+
+import 'package:matrix/matrix.dart';
+
+/// Returns a human-readable preview of the room's last event, or null if there
+/// is no suitable event to preview.
+String? lastMessagePreview(Room room) {
+  final lastEvent = room.lastEvent;
+  if (lastEvent == null) return null;
+
+  return switch (lastEvent.type) {
+    EventTypes.Message => lastEvent.body,
+    EventTypes.Encrypted => 'Encrypted message',
+    EventTypes.Sticker => 'Sticker',
+    _ => null,
+  };
+}