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.

public/configuration/ui-config.example

@@ -24,6 +24,7 @@
     "wired.action.kick.from.room.max.length": 100,
     "wired.action.mute.user.max.length": 100,
     "game.center.enabled": false,
+    "radio_ui.enabled": false,
     "guides.enabled": true,
 	"housekeeping.enabled": true,
     "toolbar.hide.quests": true,