feat(radio): make the radio widget toggleable from ui-config

Move the radio on/off switch out of renderer-config (where it sat next
to asset/data URLs) into ui-config, alongside the other UI feature
toggles (game.center.enabled, guides.enabled, …) — the natural home for
a widget switch — and rename it to the dotted convention `radio_ui.enabled`.

The MainView gate now defaults to `false`, so the radio is opt-in: an
absent key keeps it hidden; set `"radio_ui.enabled": true` in ui-config
to show it. The radio.url data source stays in renderer-config.

src/components/MainView.tsx

@@ -184,7 +184,7 @@ export const MainView: FC<{}> = props =>
             <RareValuesView />
             <FortuneWheelView />
             <SoundboardView />
-            { GetConfigurationValue<boolean>('radio_ui', true) && <RadioView /> }
+            { GetConfigurationValue<boolean>('radio_ui.enabled', false) && <RadioView /> }
             <ExternalPluginLoader />
         </>
     );