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)
