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

modules/cabana/phase10_coverage_test.go

@@ -116,6 +116,7 @@ func TestPhase10Coverage(t *testing.T) {
 			"POST /{vendor}/{plugin}/{controller}/{id}/relations/{name}/records/{child}/files/{field}/reorder",
 			"POST /{vendor}/{plugin}/{controller}/{id}/relations/{name}/unlink",
 			"PUT /account",
+			"PUT /account/preferences",
 			"PUT /settings/{code}",
 			"PUT /{vendor}/{plugin}/{controller}/{id}",
 			"PUT /{vendor}/{plugin}/{controller}/{id}/files/{field}/{file}",