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/mxc_url.dart

@@ -0,0 +1,28 @@
+// Version: 1.0.0 | Created: 2026-04-02
+// Synchronous MXC URI to HTTP URL resolution.
+// Avatars and thumbnails in the room list / sync service need a resolved HTTP
+// URL. The Matrix SDK's getDownloadUri() is async (checks authenticated media
+// support), but for avatar display we need a synchronous result.
+//
+// This helper builds the legacy v3 media URL which works on all homeservers.
+
+import 'package:matrix/matrix.dart';
+
+/// Resolves an `mxc://` [Uri] to an HTTP download URL using the client's
+/// homeserver. Returns `null` if the URI is not an mxc scheme or the client
+/// has no homeserver set.
+///
+/// This is synchronous — suitable for use in non-async model mapping.
+String? resolveMxcUrl(Client client, Uri? mxcUri) {
+  if (mxcUri == null || !mxcUri.isScheme('mxc')) return null;
+  final homeserver = client.homeserver;
+  if (homeserver == null) return null;
+
+  // Build the media download path per the Matrix spec.
+  final serverName = mxcUri.host;
+  final port = mxcUri.hasPort ? ':${mxcUri.port}' : '';
+  final mediaId = mxcUri.path; // includes leading /
+  return homeserver
+      .resolve('_matrix/media/v3/download/$serverName$port$mediaId')
+      .toString();
+}