feat(quick-261016-0fm): manual theme switch on the rail

- the rail's top button cycles system, light and dark and names the
  current choice; light and dark ignore prefers-color-scheme
- usePreferences applies the cached theme before the first await of boot,
  saves the choice through PUT /account/preferences and reverts with a
  toast when the save fails; me() applies the stored preference

admin/src/state/useSidebar.ts

@@ -2,9 +2,10 @@
 // D-06, D-10). The section panel hides below about 1100px or when the admin
 // collapses it. The viewport only forces the collapse and never overwrites
 // the admin's own choice, which persists in localStorage as a boolean under
-// SIDEBAR_KEY. The only other writer of browser storage is listState.ts,
-// which keeps list query parameters per list; tokens and record data are
-// never stored (T-10-22, T-16.1-03).
+// SIDEBAR_KEY. The other writers of browser storage are listState.ts,
+// which keeps list query parameters per list, and usePreferences.ts, which
+// caches the theme and locale codes; tokens and record data are never stored
+// (T-10-22, T-16.1-03).
 import { computed, readonly, ref } from 'vue'
 
 export const SIDEBAR_KEY = 'summer-admin.sidebar'