feat(settings/espnow): validate wifi_channel and wire into firmware

src/settings.py

@@ -45,6 +45,9 @@ class Settings(dict):
             self['session_secret_key'] = self.generate_secret_key()
             # Save immediately when generating a new key
             self.save()
+        # ESP-NOW STA channel (2.4 GHz) for LED drivers / bridge alignment; 1–11
+        if 'wifi_channel' not in self:
+            self['wifi_channel'] = 6
 
     def save(self):
         try: