fix(backend): allow renaming sessions before first message is sent

app/services/session_store.py

@@ -314,10 +314,19 @@ async def rename_session(old_id: str, new_name: str) -> str:
                     logger.info("session_store: renamed session %s -> %s", old_id, new_id)
                     return new_id
                 else:
-                    logger.warning(
-                        "session_store: cannot rename non-existent session %s", old_id
+                    logger.info(
+                        "session_store: old session %s did not exist; creating new named session %s", old_id, new_id
                     )
-                    return old_id
+                    skeleton = {
+                        "session_id": new_id,
+                        "user_name": new_name,
+                        "created_at": _now_iso(),
+                        "last_active": _now_iso(),
+                        "message_count": 0,
+                        "messages": [],
+                    }
+                    await asyncio.to_thread(_write_json, new_path, skeleton)
+                    return new_id
             except Exception as exc:
                 logger.error("session_store: failed to rename session %s: %s", old_id, exc)
                 return old_id