feat: i18n annotations — data-de/data-en + toggle for all 54 custom sites

Phase 2 of i18n rollout:
- All 54 static custom sites now have data-de/data-en attributes on
  visible text elements with English translations
- i18n.js script tag injected into all sites
- Language toggle button with machine-translation disclaimer added
  to every site's footer area
- Brand names / domain wordplay preserved unchanged in both languages
- 2 dynamic sites (dasbes.de, dumusst.com) skipped — no static HTML

sites/schulfrai.de/index.html

@@ -53,8 +53,10 @@
     <div class="icon">🎓</div>
     <h1>schul<span class='accent'>frAI</span>.</h1>
     <div class="divider"></div>
-    <p class="subtitle">Lernen ohne Grenzen.</p>
+    <p class="subtitle" data-de="Lernen ohne Grenzen." data-en="Learning without limits.">Lernen ohne Grenzen.</p>
     <p class="footer">schulfrAI.de</p>
+    <p class="footer" style="margin-top:12px;"><button data-i18n-toggle title="Maschinell übersetzt / Machine-translated — German is the original." style="background:none;border:1px solid var(--text-muted);color:var(--text-muted);font-size:0.65rem;letter-spacing:0.1em;padding:4px 12px;border-radius:4px;cursor:pointer;">EN</button><br><small data-de="Maschinell übersetzt" data-en="Machine-translated" style="font-size:0.6rem;opacity:0.5;">Maschinell übersetzt</small></p>
   </div>
+  <script src="/shared/i18n.js"></script>
 </body>
 </html>