feat: persist audio capture device selection to config.yaml

Device choice now survives server restarts. Falls back to default
if the saved device is no longer available.

media_server/config_manager.py

@@ -451,6 +451,34 @@ class ConfigManager:
                 del settings.links[name]
             logger.info(f"Link '{name}' deleted from config")
 
+    def set_setting(self, key: str, value) -> None:
+        """Set a top-level config setting and persist to YAML.
+
+        Args:
+            key: Setting name (e.g., "visualizer_device").
+            value: Setting value (None removes the key).
+        """
+        with self._lock:
+            if not self._config_path.exists():
+                data = {}
+            else:
+                with open(self._config_path, "r", encoding="utf-8") as f:
+                    data = yaml.safe_load(f) or {}
+
+            if value is None:
+                data.pop(key, None)
+            else:
+                data[key] = value
+
+            self._config_path.parent.mkdir(parents=True, exist_ok=True)
+            with open(self._config_path, "w", encoding="utf-8") as f:
+                yaml.dump(data, f, default_flow_style=False, sort_keys=False)
+
+            # Update in-memory settings
+            if hasattr(settings, key):
+                setattr(settings, key, value)
+            logger.info("Setting '%s' updated to: %s", key, value)
+
 
 # Global config manager instance
 config_manager = ConfigManager()