feat(quick-261016-0fm): per-admin theme and locale preferences

- GET and PUT /account/preferences (own row only, interactive sessions,
  CSRF header on cookie writes) store admin_theme and admin_locale in
  backend_users.metadata, keeping every other key
- GET /auth/me and the My account answers carry preferences
- phrasebook.Translator.Locales lists the locales defining a prefix
- route tables, conformance cases, OpenAPI document, TS types and docs

docs/backend/users-and-permissions.md

@@ -140,6 +140,7 @@ Every signed-in administrator can edit their own profile on the My account scree
 - **Password:** leave it blank to keep the current one. A new password needs `current_password`, at least 4 characters, at most 72 bytes (bcrypt's limit) and an equal `password_confirmation`. A wrong current password answers 422 on `current_password` and is written to the authentication log. A changed password is hashed at `admin.password.bcrypt_cost` and ends every other session: tokens issued before the change are refused. The browser that made the change gets a fresh session cookie and stays signed in; an API client that used a Bearer token has to sign in again.
 - **Subtitle:** a short line such as a role or position. It is stored where WinterCMS keeps it, under the `subtitle` key of the jsonable `backend_users.metadata` column, so rows copied from WinterCMS keep their subtitles and other metadata keys survive a save. Code reads it with `cabana.BackendUser.Subtitle`, which treats NULL, `[]` and malformed metadata as no subtitle. `GET <prefix>/api/v1/auth/me` returns it as `subtitle`.
 - **Avatar:** `POST <prefix>/api/v1/account/avatar` takes one multipart part named `file_data`. Only jpg, jpeg, png, gif and webp images that pass the image check are accepted, up to `cabana.AvatarMaxBytes` (2 MiB) and within `http.body_limits.upload_bytes`; anything else answers 422 on `avatar`, and a body over the cap answers 413. The file is stored under a name the server generates, never the uploaded file name. An upload replaces the previous avatar: the old row is deleted with the new one's save and its files right after. `DELETE <prefix>/api/v1/account/avatar` removes the avatar and answers 200 also when there is none. The avatar is a public attachOne in `system_files` with attachment type `Backend\Models\User` (`cabana.BackendUserMorphType`) and field `avatar` (`cabana.BackendUserAvatarField`), exactly as WinterCMS stores it, so avatar rows copied from WinterCMS work as they are. `GET .../auth/me` returns its `url` and a 250x250 crop `thumb_url`, and the top bar shows it in place of the initials.
+- **Preferences:** the theme (system, light or dark) and the admin language an administrator picks in the SPA are saved through `PUT <prefix>/api/v1/account/preferences` and read with `GET <prefix>/api/v1/account/preferences`; `GET .../auth/me` returns them as `preferences`. The body may hold only `theme` and `locale` (`cabana.AdminPreferencesRequest`); any other key, including `id`, answers 422 on that key and nothing is written. Each present key is saved and an absent key keeps its value, so `{}` answers the current preferences. `theme` must be `system`, `light` or `dark`; `locale` must be `""` (follow the request locale) or one of the admin locales `GET /lang` lists in `meta.locales`; another value or a non-string answers 422 on that field. They are stored under `admin_theme` and `admin_locale` in `backend_users.metadata`, next to the subtitle; the defaults (`system`, `""`) are stored as absent keys, every other key survives a save, and metadata that is not a JSON object is never overwritten (422). Like the rest of My account the routes act only on the token's own administrator and refuse personal API tokens.
 - **API tokens:** a card below the profile form (never inside it — the page Save does not touch tokens) lists, creates and revokes only the signed-in administrator's own tokens. Creating asks for a name, an expiry (`30d`, `90d`, `365d` or `never`) and scopes from `GET /account/tokens/scopes`; the plaintext is shown once in the dialog for copying, then cleared. Revoke asks for confirmation. A "Show revoked tokens" toggle reloads the list with revoked rows. See [API tokens](#api-tokens) for the wire contract and scopes.
 
 Plugins that show an administrator, such as an author box, read the subtitle with `cabana.BackendUser.Subtitle` and the picture with `cabana.BackendUserAvatar`, which returns the avatar file (or nil) for an administrator id.