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/auth/data/auth_repository.dart

@@ -1,4 +1,4 @@
-// Version: 1.0.3 | Created: 2026-04-01
+// Version: 1.1.0 | Created: 2026-04-01 | Updated: 2026-04-02
 // Auth repository: handles all Matrix login/logout API interactions.
 // Uses the Matrix Dart SDK — no raw HTTP calls for auth.
 
@@ -11,7 +11,7 @@ import '../domain/auth_failure.dart';
 
 part 'auth_repository.g.dart';
 
-@riverpod
+@Riverpod(keepAlive: true)
 AuthRepository authRepository(Ref ref) {
   return AuthRepository(client: ref.watch(matrixClientProvider));
 }
@@ -41,7 +41,7 @@ class AuthRepository {
         LoginType.mLoginPassword,
         identifier: AuthenticationUserIdentifier(user: username),
         password: password,
-        initialDeviceDisplayName: 'M8Chat',
+        initialDeviceDisplayName: AppConfig.appName,
       );
     } on MatrixException catch (e) {
       throw switch (e.errcode) {
@@ -87,7 +87,7 @@ class AuthRepository {
       newToken: accessToken,
       newUserID: userId,
       newDeviceID: deviceId,
-      newDeviceName: 'M8Chat',
+      newDeviceName: AppConfig.appName,
       newHomeserver: Uri.parse(AppConfig.matrixBaseUrl),
       newOlmAccount: null,
     );