feat(05-08): SettingsSheet (master toggle D-09) + AppNav avatar promoted to button

- Create SettingsSheet.tsx: bottom sheet (role=dialog, z:301, Escape+backdrop-close)
  with FamilySync Notifications toggle (role=switch, aria-checked, 44px target)
  wired to usePushSubscription setEnabled + permission state
  and inline permission-denied hint (AlertCircle + 'How to enable') when denied
- Promote PhoneNav avatar div to button with onOpenSettings onClick + aria-label
- Add onOpenSettings prop to DesktopNav; add avatar button at sidebar bottom
- Thread onOpenSettings through AppNavProps
- Add @keyframes spin to tokens.css (fixes missing keyframe used by SettingsSheet + SyncStateToast)

apps/pwa/src/styles/tokens.css

@@ -136,3 +136,12 @@
     background-position: 200% 0;
   }
 }
+
+@keyframes spin {
+  from {
+    transform: rotate(0deg);
+  }
+  to {
+    transform: rotate(360deg);
+  }
+}