- Replace global _last_reply float with _last_reply_per_sender dict.
A reply to one node no longer blocks all other senders for 5 s.
LRU eviction keeps the dict bounded at 200 entries.
- _get_active_channels() now falls back to BotConfig defaults when
the stored channel set is empty (user never saved a selection).
Bot was silently deaf on first run despite the panel showing all
channels pre-checked.
Closes: bot only replies to first sender in multi-node #test session.
