Add homepage and standalone pages; fix BlogNav off-state logic

Add homepage and standalone pages; fix BlogNav off-state logic

- Implement src/pages/index.astro: full BaseLayout with pack/bg/cf
  panels, 5 recent posts, and EU-funding marginalcontent aside
- Add src/pages/about.html.astro, impressum.html.astro,
  datenschutz.html.astro with content from Hugo _index.html sources
- Fix BlogNav: Archive/Categories/Tags now get 'off' class when the
  current page is not in any blog section (e.g. homepage, about page);
  previously they were only off on taxonomy-term pages
- Remove Projects/About subtree from BlogNav submenu — Hugo does not
  render them either, because the tree.html partial filters them out

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>