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/widgets/matrix_avatar.dart

@@ -0,0 +1,54 @@
+// Version: 1.0.0 | Created: 2026-04-02
+// Shared avatar widget used throughout the app. Displays a cached network
+// image when an HTTP avatar URL is available, or falls back to a coloured
+// circle with the first letter of the display name.
+//
+// The [avatarUrl] MUST be a resolved HTTP URL — never pass an mxc:// URI.
+
+import 'package:cached_network_image/cached_network_image.dart';
+import 'package:flutter/material.dart';
+
+class MatrixAvatar extends StatelessWidget {
+  const MatrixAvatar({
+    super.key,
+    required this.name,
+    this.avatarUrl,
+    this.radius = 20,
+  });
+
+  /// Display name used for the initials fallback.
+  final String name;
+
+  /// Resolved HTTP URL for the avatar image. Must NOT be an mxc:// URI.
+  final String? avatarUrl;
+
+  /// Radius of the [CircleAvatar].
+  final double radius;
+
+  @override
+  Widget build(BuildContext context) {
+    final theme = Theme.of(context);
+    final initials = name.isNotEmpty ? name[0].toUpperCase() : '?';
+
+    if (avatarUrl != null) {
+      return CircleAvatar(
+        radius: radius,
+        backgroundImage: CachedNetworkImageProvider(avatarUrl!),
+        backgroundColor: theme.colorScheme.surfaceContainerHighest,
+      );
+    }
+
+    return CircleAvatar(
+      radius: radius,
+      backgroundColor: theme.colorScheme.primary.withAlpha(51),
+      child: Text(
+        initials,
+        style: TextStyle(
+          fontSize: radius * 0.7,
+          fontWeight: FontWeight.bold,
+          color: theme.colorScheme.primary,
+        ),
+      ),
+    );
+  }
+}