feat(ui): localize theme names (#7168)

Allow translating theme names. Not even for i18n reasons but because this way the menu is clearer and cleaner.

The number of translated entries is kept minimal for now. It is easy to pollute locales with these names otherwise.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/7168
Reviewed-by: Gusted <gusted@noreply.codeberg.org>

modules/translation/mock.go

@@ -39,6 +39,10 @@ func (l MockLocale) TrSize(s int64) ReadableSize {
 	return ReadableSize{fmt.Sprint(s), ""}
 }
 
+func (l MockLocale) HasKey(key string) bool {
+	return true
+}
+
 func (l MockLocale) PrettyNumber(v any) string {
 	return fmt.Sprint(v)
 }