feat(10-02): backend strings, controller messages and declarative toolbar

- phrasebook ships the backend::lang admin strings (pl, en) with CLDR
  plural maps, loads them as namespace backend, applies
  pact.HasLangOverrides trees (lang/<locale>/<namespace>/<group>.yaml)
  after every namespace, and fails activation when a backend key cannot
  convert to plural forms
- Translator.Forms, Bundle, Resolved and Has serve keys as CLDR form maps
- Public GET /lang returns every backend::lang key for the request
  locale over the fallback locale, Cache-Control no-cache
- config_list, config_form and config_relation accept a strict messages
  block; omitted keys take framework defaults, schemas serve every message
  as CLDR forms, and activation fails on a missing phrase key
- toolbar.buttons is an ordered [create, delete] list; the Winter string
  form, duplicates, unknown actions and delete without showCheckboxes fail
  at boot, and create is dropped when the controller has no form
- Form schema serves the raw Winter redirects; scaffold emits the list
  syntax; form and relation schema routes are typed in the admin OpenAPI

cabana/admin_openapi.go

@@ -119,6 +119,20 @@ func AdminLogin() {}
 // @Router /auth/refresh [post]
 func AdminRefresh() {}
 
+// LangBundle is the public string bundle: full backend::lang key to CLDR
+// forms (D-20).
+type LangBundle map[string]MessageForms
+
+// AdminLang documents GET /lang.
+//
+// @Summary Admin UI strings
+// @Description Every backend::lang key as CLDR plural forms for the Accept-Language locale, over the fallback locale's keys. Public: the login screen loads it before signing in. meta.locale is the locale the bundle resolved to.
+// @Tags admin
+// @Produce json
+// @Success 200 {object} Envelope[LangBundle]
+// @Router /lang [get]
+func AdminLang() {}
+
 // AdminLogout documents POST /auth/logout.
 //
 // @Summary Admin logout
@@ -233,7 +247,7 @@ func AdminListSchema() {}
 // @Param vendor path string true "Vendor"
 // @Param plugin path string true "Plugin"
 // @Param controller path string true "Controller"
-// @Success 200 {object} SuccessEnvelope
+// @Success 200 {object} Envelope[FormView]
 // @Failure 401 {object} ErrorEnvelope
 // @Failure 403 {object} ErrorEnvelope
 // @Failure 404 {object} ErrorEnvelope
@@ -250,7 +264,7 @@ func AdminFormSchema() {}
 // @Param plugin path string true "Plugin"
 // @Param controller path string true "Controller"
 // @Param name path string true "Relation name"
-// @Success 200 {object} SuccessEnvelope
+// @Success 200 {object} Envelope[RelationSchema]
 // @Failure 401 {object} ErrorEnvelope
 // @Failure 403 {object} ErrorEnvelope
 // @Failure 404 {object} ErrorEnvelope