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

phrasebook/translator.go

@@ -91,6 +91,9 @@ func Activate[P interface{ ID() string }](app *backpack.App, plugins []P) error
 	if err := cat.Load("lagoon", systemLangFS); err != nil {
 		return err
 	}
+	if err := cat.Load("backend", backendLang()); err != nil {
+		return err
+	}
 	for _, p := range plugins {
 		hl, ok := any(p).(pact.HasLang)
 		if !ok {
@@ -100,6 +103,21 @@ func Activate[P interface{ ID() string }](app *backpack.App, plugins []P) error
 			return err
 		}
 	}
+	// Overrides run after every namespace is loaded (D-20): a project can
+	// replace framework or plugin strings and add locales without a rebuild
+	// of the admin SPA.
+	for _, p := range plugins {
+		ho, ok := any(p).(pact.HasLangOverrides)
+		if !ok {
+			continue
+		}
+		if err := cat.Override(p.ID(), ho.LangOverridesFS()); err != nil {
+			return err
+		}
+	}
+	if err := cat.checkForms("backend::"); err != nil {
+		return err
+	}
 	opts := Options{Locale: defaultLocale, Fallback: defaultFallback}
 	if app.Config != nil {
 		if v := strings.TrimSpace(app.Config.String("app.locale")); v != "" {
@@ -460,3 +478,102 @@ func ucfirst(s string) string {
 	}
 	return string(unicode.ToUpper(r)) + s[size:]
 }
+
+// Forms returns key's text as CLDR plural forms for the SPA (D-13, D-20): a
+// plain text is {"other": text}, a YAML plural map is copied, and a pipe of
+// category forms maps by category. Exact and range pipes ({0}, [2,*]) have
+// no Intl.PluralRules equivalent and report false, as does a missing key.
+// Placeholders are left for the client to interpolate.
+func (t *Translator) Forms(locale, key string) (map[string]string, bool) {
+	if t == nil {
+		return nil, false
+	}
+	e, _, ok := t.find(locale, key)
+	if !ok {
+		return nil, false
+	}
+	return entryForms(e)
+}
+
+// Bundle returns every key starting with prefix as CLDR forms, keys of the
+// requested locale over those of its fallback chain. Keys that cannot convert
+// are left out.
+func (t *Translator) Bundle(locale, prefix string) map[string]map[string]string {
+	out := map[string]map[string]string{}
+	if t == nil || t.cat == nil {
+		return out
+	}
+	chain := fallbackChain(locale, t.fallbackLocale())
+	for i := len(chain) - 1; i >= 0; i-- {
+		for key, e := range t.cat.messages[chain[i]] {
+			if !strings.HasPrefix(key, prefix) {
+				continue
+			}
+			if forms, ok := entryForms(e); ok {
+				out[key] = forms
+			}
+		}
+	}
+	return out
+}
+
+// Resolved returns the first locale of locale's fallback chain that the
+// catalog has any key for under prefix: the locale a Bundle is mostly in.
+func (t *Translator) Resolved(locale, prefix string) string {
+	if t == nil || t.cat == nil {
+		return locale
+	}
+	for _, step := range fallbackChain(locale, t.fallbackLocale()) {
+		for key := range t.cat.messages[step] {
+			if strings.HasPrefix(key, prefix) {
+				return step
+			}
+		}
+	}
+	return t.fallbackLocale()
+}
+
+// Has reports whether any locale of the catalog defines key.
+func (t *Translator) Has(key string) bool {
+	if t == nil || t.cat == nil {
+		return false
+	}
+	for _, byKey := range t.cat.messages {
+		if _, ok := byKey[key]; ok {
+			return true
+		}
+	}
+	return false
+}
+
+func (t *Translator) fallbackLocale() string {
+	if t == nil || strings.TrimSpace(t.fallback) == "" {
+		return defaultFallback
+	}
+	return t.fallback
+}
+
+func entryForms(e entry) (map[string]string, bool) {
+	switch {
+	case len(e.pipes) > 0:
+		out := make(map[string]string, len(e.pipes))
+		for _, part := range e.pipes {
+			if part.exact != nil || part.lo != nil || part.cat == "" {
+				return nil, false
+			}
+			out[part.cat] = part.text
+		}
+		if _, ok := out["other"]; !ok {
+			return nil, false
+		}
+		return out, true
+	case e.plurals != nil:
+		out := make(map[string]string, len(e.plurals))
+		for cat, text := range e.plurals {
+			out[cat] = text
+		}
+		return out, true
+	default:
+		return map[string]string{"other": e.text}, true
+	}
+}