feat(ui): implement premium beige design system and ux refinements

backend/app/conversation_tracker.py

@@ -9,7 +9,7 @@ if TYPE_CHECKING:
 
 _ENSURE_SQL = """
 INSERT INTO conversations
-    (thread_id, started_at, last_activity)
+    (thread_id, created_at, last_activity)
 VALUES
     (%(thread_id)s, NOW(), NOW())
 ON CONFLICT (thread_id) DO NOTHING