feat(gallery): grid title, subtle download icon, full i18n

- Render block.title as <h3> above the grid when set; the carousel
  variant is unchanged.
- Download CTA in the modal shrunk to a 9×9 icon-only button with
  text-white/80 hover-tint instead of the dark green link colour the
  app's `main a` rule was forcing in. Adds `no-underline` so the
  global underline doesn't sneak back in.
- All previously hardcoded German strings (Download, Schließen,
  modal aria, open aria) now go through translations.ts → CMS
  translation_bundle. Cms_content app.json5 ships the matching keys.

Open follow-up: per-image title/alt/caption is empty on the existing
windwiderstand mediathek gallery → editor needs to fill those if
hover labels are wanted.

src/lib/translations.ts

@@ -94,6 +94,10 @@ const TRANSLATION_KEYS = [
   "image_gallery_position_aria",
   "image_gallery_slide_aria",
   "image_gallery_empty",
+  "image_gallery_download",
+  "image_gallery_close",
+  "image_gallery_open_aria",
+  "image_gallery_modal_aria",
   "youtube_missing",
   "youtube_title_fallback",
   "calendar_prev_month",