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/main.tsx

@@ -9,6 +9,7 @@ import { PlaylistsProvider } from "./components/PlaylistsProvider";
 import { PlexProvider } from "./components/PlexProvider";
 import { ChannelsProvider } from "./components/ChannelsProvider";
 import { FeedViewProvider } from "./components/FeedViewProvider";
+import { PrefsProvider } from "./components/PrefsProvider";
 import { WizardProvider } from "./components/WizardProvider";
 import "./i18n";
 import "./index.css";
@@ -48,9 +49,11 @@ const root =
                 <PlexProvider>
                   <ChannelsProvider>
                     <FeedViewProvider>
-                      <WizardProvider>
-                        <App />
-                      </WizardProvider>
+                      <PrefsProvider>
+                        <WizardProvider>
+                          <App />
+                        </WizardProvider>
+                      </PrefsProvider>
                     </FeedViewProvider>
                   </ChannelsProvider>
                 </PlexProvider>