feat(appearance): opt-in glass tuner toggle in Settings

Graduate the floating glass tuner from a localhost-only dev tool to a
user-facing opt-in: it's now gated on a per-user `theme.showTuner` pref (off by
default) with a "Show the glass tuner" switch in Settings → Appearance, riding
the existing theme draft-save. Softened the dev-only framing. Behaviour and look
are unchanged; the full preset system is a separate epic.

frontend/src/App.tsx

@@ -985,8 +985,8 @@ export default function App() {
       <ErrorDialog />
       {/* Re-binds to the active page's scroll container on navigation (page or channel change). */}
       <BackToTop dep={channelView ? `chan:${channelView.id}` : page} />
-      {/* Local-only live appearance tuner (renders only on localhost; null everywhere else). */}
-      <GlassTuner />
+      {/* Opt-in live appearance tuner (Settings → Appearance; off by default). */}
+      <GlassTuner enabled={theme.showTuner} />
     </div>
   );
 }