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/main.ts

@@ -4,15 +4,17 @@ import { createAdminRouter } from './app/router'
 import { onUnauthorized } from './api/client'
 import { loadFieldTypes } from './app/fieldTypes'
 import { loadStrings } from './app/i18n'
-import { applyColorScheme } from './app/theme'
 import { expireSession, me } from './state/useAuth'
 import { loadNavigation } from './state/useNavigation'
+import { applyTheme, cachedTheme } from './state/usePreferences'
 import { loadSettings } from './state/useSettings'
 import './styles/main.css'
 
 async function boot(): Promise<void> {
-  // Dark mode follows the system preference before anything renders (A6).
-  applyColorScheme()
+  // The last theme choice (system unless the admin picked light or dark) is
+  // applied before the first await, so nothing renders in the wrong theme;
+  // me() then applies the admin's stored preference.
+  applyTheme(cachedTheme())
   // Boot order (research Pattern 7): the public strings bundle first, so the
   // login screen is translated, then the session, then navigation and the
   // plugin field types.