Add public changelog with Atom feed and What's New indicator

- Changelog entries authored as markdown files in apps/journal/changelog/
  with YAML frontmatter, loaded via Vite import.meta.glob at build time
- /changelog list page with date, title, and preview
- /changelog/:slug detail page with full markdown rendering (react-markdown)
  and Open Graph meta tags for social sharing
- /changelog/feed.xml Atom feed with auto-discovery <link> tags
- "What's New" dot on nav bar using localStorage timestamp tracking
- Notification toggle: users can permanently disable/re-enable the indicator
- i18n strings for en and de
- Initial changelog entry covering the Phase 1 launch

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

package.json

@@ -66,6 +66,7 @@
     "react-dom": "^19.2.4",
     "react-i18next": "^17.0.2",
     "react-leaflet": "^5.0.0",
+    "react-markdown": "^10.1.0",
     "react-router": "catalog:",
     "tailwindcss": "^4.2.2",
     "turbo": "^2.9.3",