website: fix HTML issues, mobile nav, i18n, and SEO

Bug Fixes:
- index.html: remove duplicate <title> tags (invalid HTML), use single
  title + JS swap; fix og:image relative→absolute URL (logo.png)
- join.html: fix hardcoded lang=de, add language detection IIFE,
  add lang toggle to nav, make badge/title/desc/actions bilingual,
  add missing </head> closing tag, add noindex meta
- app.js: join status handler was German-only; now fully bilingual
  (Success/Error titles, countdown, close/retry button labels)

Improvements:
- Add Firefox 'Add to Firefox' button to hero CTA (placeholder href=#)
- impressum.html / datenschutz.html: add noindex meta to keep legal
  pages out of search results
- robots.txt: add Sitemap directive for SEO crawlability
- sitemap.xml: new file listing main page + legal pages with priorities
- style.css: mobile nav hamburger menu (☰) replaces display:none
  blackout; nav-links drop down on mobile with backdrop blur
- style.css: cta-group flex-wrap for narrow screens
- Hamburger button added to all 4 page navs

website/style.css

@@ -399,9 +399,26 @@ footer {
     }
     .cta-group {
         justify-content: center;
+        flex-wrap: wrap;
     }
     .nav-links {
         display: none;
+        position: absolute;
+        top: 100%;
+        left: 0;
+        right: 0;
+        flex-direction: column;
+        background: rgba(15, 23, 42, 0.95);
+        backdrop-filter: blur(12px);
+        padding: 1rem 2rem;
+        gap: 1rem;
+        border-bottom: 1px solid var(--glass-border);
+    }
+    .nav-links.open {
+        display: flex;
+    }
+    .hamburger {
+        display: flex;
     }
     .legal-card {
         padding: 1.5rem;
@@ -425,3 +442,17 @@ html.lang-de [lang="en"] {
 .lang-toggle:hover {
     color: var(--accent);
 }
+
+/* --- Hamburger Menu --- */
+.hamburger {
+    display: none;
+    align-items: center;
+    justify-content: center;
+    background: none;
+    border: none;
+    color: var(--text);
+    font-size: 1.5rem;
+    cursor: pointer;
+    padding: 0.25rem;
+    line-height: 1;
+}