fix(chat): render message timestamps in device-local time

Live chat bubbles read createdAt.hour/.minute directly, but server created_at (UTC, ISO-Z) was parsed without .toLocal() while optimistic sends used DateTime.now() (local). On any non-UTC device, your own messages showed local time and received/history messages showed UTC within the same conversation. Add .toLocal() at the history-load + incoming-WS parse sites in both apps so bubbles match the optimistic path and the transcript view. Session timer math was already tz-safe (Dart .difference uses absolute instants).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

mitra_app/lib/core/chat/mitra_chat_notifier.dart

@@ -214,7 +214,9 @@ class MitraChat extends _$MitraChat {
         content: m['content'] as String,
         type: m['type'] as String? ?? MessageType.text,
         status: m['status'] as String? ?? MessageStatus.sent,
-        createdAt: DateTime.parse(m['created_at'] as String),
+        // Server sends UTC (ISO-8601 with Z); render in device-local time so
+        // bubbles match optimistic sends (DateTime.now()) + the transcript view.
+        createdAt: DateTime.parse(m['created_at'] as String).toLocal(),
       )).toList();
 
       final token = ref.read(authBridgeProvider).accessToken;
@@ -329,7 +331,8 @@ class MitraChat extends _$MitraChat {
           content: data['content'] as String,
           type: data['message_type'] as String? ?? MessageType.text,
           status: MessageStatus.sent,
-          createdAt: DateTime.parse(data['created_at'] as String),
+          // UTC from server → device-local for display (see history-load note).
+          createdAt: DateTime.parse(data['created_at'] as String).toLocal(),
         );
         state = current.copyWith(messages: [...current.messages, msg]);
         markDelivered([msg.id]);