refactor(prefs): move Settings auto-save machinery into PrefsProvider

Lift the four editable Settings prefs (theme/perf/hints/notifications), their
live-apply effects, the debounced auto-save + baseline echo-guard, and the
server-adopt-on-login into PrefsProvider, exposing the PrefsController via
usePrefs(). SettingsPanel and GlassTuner read it from the hook instead of props;
App's big adopt effect keeps only the shell prefs (panel layouts, rail collapse,
language). App.tsx 655->489 lines. E2E 17/17 incl. settings auto-save. (Platform S6b.3)

frontend/src/components/GlassTuner.tsx

@@ -1,5 +1,6 @@
 import { useEffect, useMemo, useState } from "react";
 import Overlay from "./Overlay";
+import { usePrefs } from "./PrefsProvider";
 
 /**
  * A floating live appearance tuner. Mounted at the app root so it's reachable on every page; it
@@ -91,7 +92,9 @@ function loadSaved(): Saved {
   }
 }
 
-export default function GlassTuner({ enabled }: { enabled: boolean }) {
+export default function GlassTuner() {
+  // Opt-in via Settings → Appearance (theme.showTuner, off by default); the flag lives in PrefsProvider.
+  const enabled = usePrefs().theme.showTuner;
   const initial = useMemo(loadSaved, []);
   const [vars, setVars] = useState<Record<string, number>>(() => {
     const base: Record<string, number> = {};