feat(footer): hideable legal links + socials + promo banner (#441 + #440)

Combined footer overhaul:

- Per-CMS-page show_in_footer toggle (#441) — admins can hide
  Impressum / Datenschutz from the gallery footer when an external
  privacy / imprint URL is enough.
- Five social-media URL fields in branding settings (#441) — Facebook,
  Instagram, WhatsApp, X/Twitter, YouTube. Empty string hides each
  icon individually; the row is omitted when none are set.
- Promotional banner slot above or below the gallery footer (#440) —
  global default authored as markdown in branding settings, plus a
  three-way per-event override on the Edit Event form
  (inherit / custom / off). Backend nulls promo_markdown automatically
  when mode != 'custom' so stale text never persists.

Sanitization: marked with gfm/breaks → DOMPurify with a tight
allowlist (no img, no tables, no inline html). Post-process forces
target=_blank rel="noopener noreferrer nofollow" on every link so
admin-set URLs can't tab-nap the gallery context.

i18n covers all six locales (en/de/nl/pt/ru/fr).

Targets the beta branch.

frontend/package.json

@@ -40,6 +40,7 @@
     "lodash": "^4.17.21",
     "lowlight": "^2.9.0",
     "lucide-react": "0.525.0",
+    "marked": "^15.0.12",
     "photoswipe": "^5.4.4",
     "react": "^18.3.1",
     "react-countdown": "^2.3.5",