fix(auth): always seed the active account into the switch list

Sessions created before multi-session existed had no account_ids, so adding a second
account left the first one (e.g. the long-lived session) absent from the switcher.
current_user now ensures the active user_id is always present in account_ids, so the
switch list is never missing the account you're using.

backend/app/auth.py

@@ -232,6 +232,12 @@ def current_user(request: Request, db: Session = Depends(get_db)) -> User:
     if user is None:
         request.session.clear()
         raise HTTPException(status_code=401, detail="Not authenticated")
+    # Always keep the active account in the switchable list — covers sessions created before
+    # multi-session existed (their account_ids was never seeded), so the switcher isn't empty.
+    accounts = request.session.get("account_ids") or []
+    if user_id not in accounts:
+        accounts.append(user_id)
+        request.session["account_ids"] = accounts[-MAX_SESSION_ACCOUNTS:]
     return user