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/SettingsPanel.tsx

@@ -12,6 +12,7 @@ import { Section, SettingRow, Switch } from "./ui/form";
 import { type SaveState } from "./ui/DraftSaveBar";
 import { useConfirm } from "./ConfirmProvider";
 import { useWizard } from "./WizardProvider";
+import { usePrefs } from "./PrefsProvider";
 
 // The Settings page edits server-persisted preferences. Each change applies locally for instant
 // preview AND auto-saves (debounced) — no draft, no Save button; a small indicator flashes the
@@ -40,8 +41,10 @@ const TABS: { id: TabId; icon: typeof Monitor }[] = [
 // Settings as a page (Design B): rendered in the main content area, not an overlay. It keeps
 // the frosted `.glass` surface so it still refracts the ambient backdrop. The page title is
 // shown by the Header; the tab rail stays as in-page sub-navigation.
-export default function SettingsPanel({ me, prefs }: { me: Me; prefs: PrefsController }) {
+export default function SettingsPanel({ me }: { me: Me }) {
   const { t } = useTranslation();
+  // The editable prefs (theme/perf/hints/notifications + save state) live in PrefsProvider.
+  const prefs = usePrefs();
   // "Connect YouTube" onboarding lives in WizardProvider (the Account tab's reconnect button).
   const { openWizard } = useWizard();
   const [tabRaw, setTab] = useAccountPersistedState(LS.settingsTab, "appearance");