Two bugs found during review of the previous FSM state restore fix:
1. Re-entering promo flow created nested _prev_data (unbounded growth).
Now skips save if already in PromoCodeStates.waiting_for_code and
strips _prev_ keys from saved data to prevent nesting.
2. _restore_previous_state used `if prev_state:` which treated saved
None state (user at menu) same as "no saved state". Now uses a
sentinel to distinguish the two cases, correctly restoring None
state with its data instead of calling state.clear().
Fringg
b607993854