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/datenschutz.html

@@ -6,6 +6,7 @@
     <title>Datenschutz / Privacy Policy | KoalaSync</title>
     <link rel="stylesheet" href="style.css">
     <link rel="icon" type="image/png" href="assets/logo.png">
+    <meta name="robots" content="noindex">
     <script>
         (function() {
             var savedLang = localStorage.getItem('koala_lang');
@@ -40,6 +41,7 @@
                 <img src="assets/logo.png" alt="KoalaSync Logo">
                 <span>KoalaSync</span>
             </a>
+            <button class="hamburger" onclick="document.querySelector('.nav-links').classList.toggle('open')" aria-label="Menu">☰</button>
             <div class="nav-links">
                 <a href="index.html"><span lang="de">Startseite</span><span lang="en">Home</span></a>
                 <a href="https://github.com/shik3i/KoalaSync" target="_blank">GitHub</a>