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

backend/app/ws_handler.py

@@ -54,7 +54,10 @@ async def handle_user_message(
     interrupt_manager: InterruptManager | None = None,
 ) -> None:
     """Process a user message through the graph and stream results back."""
-    if session_manager.is_expired(thread_id):
+    # Touch first so new sessions are created before expiry check.
+    # For existing sessions, touch resets the sliding window.
+    existing = session_manager.get_state(thread_id)
+    if existing is not None and session_manager.is_expired(thread_id):
         msg = "Session expired. Please start a new conversation."
         await _send_json(ws, {"type": "error", "message": msg})
         return