fix(bot): per-sender cooldown + empty-channel fallback (v1.20.1)

- 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.

meshcore_gui/services/cache.py

@@ -167,6 +167,41 @@ class DeviceCache:
             self._data["channel_keys"] = keys
             self.save()
 
+    # ------------------------------------------------------------------
+    # Channel names
+    # ------------------------------------------------------------------
+
+    def get_channel_names(self) -> Dict[int, str]:
+        """Return cached channel names as ``{idx: name}``.
+
+        Always available regardless of ``CHANNEL_CACHE_ENABLED``.
+        Keys are returned as integers for direct use as channel indices.
+        """
+        raw: Dict[str, str] = self._data.get("channel_names", {})
+        result: Dict[int, str] = {}
+        for k, v in raw.items():
+            try:
+                result[int(k)] = v
+            except (ValueError, TypeError):
+                pass
+        return result
+
+    def set_channel_names(self, names: Dict[int, str]) -> None:
+        """Store a complete channel-name mapping and persist to disk.
+
+        Replaces any previously cached names with the supplied mapping.
+        Intended to be called after every successful channel discovery so
+        the most recent names are always available at next startup,
+        independent of ``CHANNEL_CACHE_ENABLED``.
+
+        Args:
+            names: Mapping of channel index to channel name,
+                   e.g. ``{0: "Public", 1: "#localmesh"}``.
+        """
+        self._data["channel_names"] = {str(k): v for k, v in names.items()}
+        self.save()
+        debug_print(f"Cache: channel names saved: {names}")
+
     # ------------------------------------------------------------------
     # Contacts (merge strategy)
     # ------------------------------------------------------------------