test(quick-261016-0fm): admin preference coverage

- Translator.Locales: sorted, prefix-scoped, nil-safe
- preferencesOf, validPreferences, refused keys, metadata writes and
  principalFrom's PreferredLocale (internal, Docker-free)
- Docker: refused keys and values leave the row alone, owner-only saves,
  personal tokens get interactive_only and keep the request locale, the
  cookie CSRF header, non-object metadata stays byte-identical

modules/phrasebook/translator_test.go

@@ -343,3 +343,31 @@ func TestCatalogBoundaries(t *testing.T) {
 		}
 	})
 }
+
+func TestTranslatorLocales(t *testing.T) {
+	t.Parallel()
+
+	fsys := fstest.MapFS{
+		"lang/pl/lang.yaml":  {Data: []byte("greeting: Cześć\n")},
+		"lang/en/lang.yaml":  {Data: []byte("greeting: Hello\n")},
+		"lang/de/other.yaml": {Data: []byte("greeting: Hallo\n")},
+	}
+	cat := NewCatalog()
+	if err := cat.Load("acme.blog", fsys); err != nil {
+		t.Fatalf("Load: %v", err)
+	}
+	tr := NewTranslator(cat, Options{})
+	if got := tr.Locales("acme.blog::lang."); strings.Join(got, ",") != "en,pl" {
+		t.Fatalf("Locales(lang) = %v, want [en pl]", got)
+	}
+	if got := tr.Locales("acme.blog::"); strings.Join(got, ",") != "de,en,pl" {
+		t.Fatalf("Locales(namespace) = %v, want [de en pl]", got)
+	}
+	if got := tr.Locales("acme.shop::"); len(got) != 0 {
+		t.Fatalf("Locales(unknown) = %v, want none", got)
+	}
+	var nilTr *Translator
+	if got := nilTr.Locales("acme.blog::"); got != nil {
+		t.Fatalf("nil translator Locales = %v, want nil", got)
+	}
+}