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/useAuth.ts

@@ -5,6 +5,7 @@ import type { RouteLocationNormalizedLoaded, Router } from 'vue-router'
 import { api, onRefreshed, refreshSession } from '../api/client'
 import type { AdminProfile } from '../api/types'
 import { setListStateOwner } from './listState'
+import { applyProfilePreferences } from './usePreferences'
 import { setNavigation } from './useNavigation'
 import { setSettings } from './useSettings'
 
@@ -52,11 +53,13 @@ export async function login(identifier: string, password: string): Promise<boole
  * Loads the signed-in admin; null when the session is missing or expired.
  * The stored list queries follow the answer: a profile keeps only that
  * admin's (another admin's are removed, T-16.1-03), no profile removes all.
+ * The profile's UI preferences (theme) are applied before it returns.
  */
 export async function me(): Promise<AdminProfile | null> {
   const { data, response } = await api.GET('/auth/me')
   user.value = response.ok && data ? data.data : null
   setListStateOwner(user.value?.id ?? null)
+  await applyProfilePreferences(user.value)
   return user.value
 }