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/calls/presentation/incoming_call_overlay.dart

@@ -1,13 +1,13 @@
-// Version: 1.1.0 | Created: 2026-04-01
+// Version: 1.2.0 | Created: 2026-04-01 | Updated: 2026-04-02
 // IncomingCallOverlay — full-screen overlay shown when an m.call.invite
 // arrives. Displays caller name/avatar, and Accept / Decline buttons.
 
-import 'package:cached_network_image/cached_network_image.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 import 'package:go_router/go_router.dart';
 import 'package:riverpod_annotation/riverpod_annotation.dart';
 
+import '../../../shared/widgets/matrix_avatar.dart';
 import '../data/matrixrtc_repository.dart';
 import '../domain/incoming_call.dart';
 
@@ -58,14 +58,24 @@ class IncomingCallOverlayHost extends ConsumerWidget {
   }
 }
 
-class _IncomingCallOverlay extends ConsumerWidget {
+class _IncomingCallOverlay extends StatefulWidget {
   const _IncomingCallOverlay({required this.call});
 
   final IncomingCall call;
 
   @override
-  Widget build(BuildContext context, WidgetRef ref) {
+  State<_IncomingCallOverlay> createState() => _IncomingCallOverlayState();
+}
+
+class _IncomingCallOverlayState extends State<_IncomingCallOverlay> {
+  bool _dismissed = false;
+
+  @override
+  Widget build(BuildContext context) {
+    if (_dismissed) return const SizedBox.shrink();
+
     final theme = Theme.of(context);
+    final call = widget.call;
 
     return Positioned.fill(
       child: Material(
@@ -75,7 +85,13 @@ class _IncomingCallOverlay extends ConsumerWidget {
             mainAxisAlignment: MainAxisAlignment.center,
             children: [
               // Caller avatar
-              _CallerAvatar(call: call),
+              MatrixAvatar(
+                name: call.callerDisplayName.isNotEmpty
+                    ? call.callerDisplayName
+                    : call.callerId,
+                avatarUrl: call.callerAvatarUrl,
+                radius: 56,
+              ),
               const SizedBox(height: 24),
 
               // Caller name
@@ -106,10 +122,7 @@ class _IncomingCallOverlay extends ConsumerWidget {
                     icon: Icons.call_end,
                     label: 'Decline',
                     colour: Colors.red,
-                    onTap: () {
-                      // Dismiss the overlay by navigating away; the repository
-                      // stream will emit null on the next event cycle.
-                    },
+                    onTap: () => setState(() => _dismissed = true),
                   ),
                   _CallActionButton(
                     icon: call.isVideo ? Icons.videocam : Icons.call,
@@ -131,39 +144,6 @@ class _IncomingCallOverlay extends ConsumerWidget {
   }
 }
 
-class _CallerAvatar extends StatelessWidget {
-  const _CallerAvatar({required this.call});
-
-  final IncomingCall call;
-
-  @override
-  Widget build(BuildContext context) {
-    final initials = call.callerDisplayName.isNotEmpty
-        ? call.callerDisplayName[0].toUpperCase()
-        : '?';
-
-    if (call.callerAvatarUrl != null) {
-      return CircleAvatar(
-        radius: 56,
-        backgroundImage: CachedNetworkImageProvider(call.callerAvatarUrl!),
-      );
-    }
-
-    return CircleAvatar(
-      radius: 56,
-      backgroundColor: Theme.of(context).colorScheme.primary.withAlpha(77),
-      child: Text(
-        initials,
-        style: const TextStyle(
-          fontSize: 40,
-          color: Colors.white,
-          fontWeight: FontWeight.bold,
-        ),
-      ),
-    );
-  }
-}
-
 class _CallActionButton extends StatelessWidget {
   const _CallActionButton({
     required this.icon,