Add Impressum page (§ 5 DDG) and footer link

Legally required Impressum for the Einzelunternehmen (Kleinunternehmer,
no VAT ID, not in Handelsregister). Covers § 5 DDG contact data,
§ 18 Abs. 2 MStV responsible person, § 36 VSBG, and liability/copyright
disclaimers. Linked from the footer on every page.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/components/Footer.astro

@@ -6,7 +6,7 @@ const year = new Date().getFullYear();
 
 <footer class="site-footer">
   <div class="site-footer__inner">
-    <p>© {year} {SITE.author}</p>
+    <p>© {year} {SITE.author} · <a href="/impressum">Impressum</a></p>
     {
       SOCIALS.length > 0 && (
         <nav aria-label="Social" class="socials">