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/matrix_id.dart

@@ -0,0 +1,16 @@
+// Version: 1.0.0 | Created: 2026-04-02
+// Utility extension for extracting the localpart from a Matrix user ID.
+// Matrix IDs have the form @localpart:server.name — this extracts "localpart".
+
+/// Extension on [String] for Matrix user ID operations.
+extension MatrixIdExtension on String {
+  /// Extracts the localpart from a Matrix user ID.
+  ///
+  /// Example: `'@alice:matrix.m8chat.au'.matrixLocalpart` returns `'alice'`.
+  /// Returns the original string unchanged if it does not match the expected
+  /// `@localpart:server` format.
+  String get matrixLocalpart {
+    final value = split(':').first.replaceFirst('@', '');
+    return value.isEmpty ? this : value;
+  }
+}