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/loader.go

@@ -19,6 +19,8 @@ type Catalog struct {
 	messages map[string]map[string]entry
 	// namespace (plugin ID) -> owning plugin ID
 	namespaces map[string]string
+	// replace lets put overwrite an existing key; only Override sets it.
+	replace bool
 }
 
 type entry struct {
@@ -221,7 +223,7 @@ func (c *Catalog) put(pluginID, file, locale, key string, e entry) error {
 		byKey = make(map[string]entry)
 		c.messages[locale] = byKey
 	}
-	if _, exists := byKey[key]; exists {
+	if _, exists := byKey[key]; exists && !c.replace {
 		return catalogErrorf(pluginID, file, key, "duplicate key")
 	}
 	byKey[key] = e
@@ -240,6 +242,106 @@ func (c *Catalog) lookup(locale, key string) (entry, bool) {
 	return e, ok
 }
 
+// Override applies owner's override tree, laid out as
+// lang/<locale>/<namespace>/<group>.yaml, over the loaded catalog. It may
+// replace existing keys of a loaded namespace and add locales; a malformed
+// path or an unknown namespace fails.
+func (c *Catalog) Override(owner string, fsys fs.FS) error {
+	if c == nil {
+		return fmt.Errorf("phrasebook: catalog is nil")
+	}
+	if fsys == nil {
+		return nil
+	}
+	var files []string
+	err := fs.WalkDir(fsys, ".", func(p string, d fs.DirEntry, err error) error {
+		if err != nil {
+			return err
+		}
+		if !d.IsDir() {
+			files = append(files, p)
+		}
+		return nil
+	})
+	if err != nil {
+		return catalogErrorf(owner, "", "", "walk lang overrides: %s", err.Error())
+	}
+	sort.Strings(files)
+	c.replace = true
+	defer func() { c.replace = false }()
+	for _, file := range files {
+		locale, namespace, group, err := parseOverridePath(file)
+		if err != nil {
+			return catalogErrorf(owner, file, "", "%s", err.Error())
+		}
+		if _, loaded := c.namespaces[namespace]; !loaded {
+			return catalogErrorf(owner, file, "", "override names unknown namespace %s", namespace)
+		}
+		raw, err := fs.ReadFile(fsys, file)
+		if err != nil {
+			return catalogErrorf(owner, file, "", "read: %s", err.Error())
+		}
+		if err := c.loadFile(namespace, file, locale, group, raw); err != nil {
+			return fmt.Errorf("phrasebook: override by %s: %w", owner, err)
+		}
+	}
+	return nil
+}
+
+func parseOverridePath(p string) (locale, namespace, group string, err error) {
+	bad := fmt.Errorf("malformed override path (want lang/<locale>/<namespace>/<group>.yaml)")
+	if strings.Contains(p, "\\") || path.Clean(p) != p {
+		return "", "", "", bad
+	}
+	parts := strings.Split(p, "/")
+	if len(parts) != 4 || parts[0] != "lang" || !strings.HasSuffix(parts[3], ".yaml") {
+		return "", "", "", bad
+	}
+	locale, namespace, group = parts[1], parts[2], strings.TrimSuffix(parts[3], ".yaml")
+	if !validLocale(locale) || !validNamespace(namespace) || !validGroup(group) {
+		return "", "", "", bad
+	}
+	return locale, namespace, group, nil
+}
+
+func validNamespace(s string) bool {
+	if s == "" || s == "." || s == ".." || strings.HasPrefix(s, ".") || strings.HasSuffix(s, ".") {
+		return false
+	}
+	for _, r := range s {
+		if unicode.IsLetter(r) || unicode.IsDigit(r) || r == '_' || r == '-' || r == '.' {
+			continue
+		}
+		return false
+	}
+	return true
+}
+
+// checkForms fails when a key under prefix cannot convert to CLDR forms, so
+// strings served to the admin SPA never need Laravel exact or range pipes.
+func (c *Catalog) checkForms(prefix string) error {
+	locales := make([]string, 0, len(c.messages))
+	for locale := range c.messages {
+		locales = append(locales, locale)
+	}
+	sort.Strings(locales)
+	for _, locale := range locales {
+		keys := make([]string, 0)
+		for key := range c.messages[locale] {
+			if strings.HasPrefix(key, prefix) {
+				keys = append(keys, key)
+			}
+		}
+		sort.Strings(keys)
+		for _, key := range keys {
+			if _, ok := entryForms(c.messages[locale][key]); !ok {
+				return fmt.Errorf("phrasebook: %s key %s cannot convert to CLDR plural forms (use a plural map, not an exact or range pipe)", locale, key)
+			}
+		}
+	}
+	return nil
+}
+
 func parseLangPath(p string) (locale, group string, err error) {
 	p = path.Clean("/" + strings.ReplaceAll(p, "\\", "/"))
 	p = strings.TrimPrefix(p, "/")