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/src/components/common/MarkdownContent.tsx

@@ -0,0 +1,68 @@
+import React, { useMemo } from 'react';
+import { marked } from 'marked';
+import DOMPurify from 'dompurify';
+
+interface MarkdownContentProps {
+  /** Raw markdown source. Empty/null → renders nothing. */
+  source?: string | null;
+  /** Wrapper className. Default lets the consumer style spacing. */
+  className?: string;
+}
+
+// Sanitization allowlist — kept tight on purpose. The promo slot is
+// admin-authored content rendered to gallery visitors, so the surface
+// has to be conservative. Markdown source means contributors don't
+// hand-write HTML in the first place; this is defense-in-depth.
+const ALLOWED_TAGS = [
+  'p', 'br', 'strong', 'em', 'b', 'i', 'u', 'del', 's',
+  'a', 'ul', 'ol', 'li', 'blockquote',
+  'h1', 'h2', 'h3', 'h4', 'h5', 'h6',
+  'code', 'pre',
+];
+const ALLOWED_ATTR = ['href', 'title', 'target', 'rel'];
+
+// Marked configuration:
+//   gfm: true          → autolinks, strikethrough, tables (we don't allow tables in ALLOWED_TAGS so they sanitize out cleanly)
+//   breaks: true       → newline = <br> (matches what an admin types in a textarea)
+//   pedantic: false    → modern interpretation of edge cases
+marked.setOptions({ gfm: true, breaks: true, pedantic: false });
+
+/**
+ * Render admin-authored markdown safely. Used by the gallery footer
+ * promotional slot (#440) and any future admin-content surface that
+ * needs more than plain text but less than a full WYSIWYG editor.
+ *
+ * Pipeline: marked.parse → DOMPurify with the allowlist above. Returns
+ * null when the source is empty so callers can use it inline without
+ * a wrapper-when-empty problem.
+ */
+export const MarkdownContent: React.FC<MarkdownContentProps> = ({ source, className }) => {
+  const html = useMemo(() => {
+    const md = (source ?? '').trim();
+    if (!md) return '';
+    const raw = marked.parse(md, { async: false }) as string;
+    return DOMPurify.sanitize(raw, {
+      ALLOWED_TAGS,
+      ALLOWED_ATTR,
+      ALLOW_DATA_ATTR: false,
+      KEEP_CONTENT: true,
+      // Force external links to noopener/noreferrer so admin-set URLs
+      // can't tab-nap the gallery context.
+      ADD_ATTR: ['target', 'rel'],
+    });
+  }, [source]);
+
+  if (!html) return null;
+  return (
+    <div
+      className={className}
+      // Add target=_blank + rel safety to all <a> after sanitize. Doing
+      // this with a hook would be cleaner but DOMPurify lacks a generic
+      // "set attribute on tag" hook in this version, so post-process.
+      dangerouslySetInnerHTML={{ __html: html.replace(
+        /<a /g,
+        '<a target="_blank" rel="noopener noreferrer nofollow" '
+      ) }}
+    />
+  );
+};

frontend/src/components/common/index.ts

@@ -23,3 +23,4 @@ export { ProtectedImage } from './ProtectedImage';
 export { ProtectionWarning } from './ProtectionWarning';
 export { ReCaptcha } from './ReCaptcha';
 export { PasswordGenerator } from './PasswordGenerator';
+export { MarkdownContent } from './MarkdownContent';