feat: add diarization section to settings page

Adds a "Diarisierung" section with an enabled/disabled toggle,
HuggingFace token input, and a help link to pyannote/speaker-diarization-3.1.
loadConfig() and the save handler now persist diarization settings.

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

frontend/settings.html

@@ -94,6 +94,25 @@
         <button class="btn primary" id="save-btn">Speichern</button>
       </div>
     </section>
+
+    <section>
+      <h2>Diarisierung</h2>
+      <div class="field">
+        <label>
+          <input type="checkbox" id="diar-enabled" style="margin-right:6px;accent-color:var(--yellow);">
+          Sprecher-Erkennung aktivieren
+        </label>
+      </div>
+      <div class="field">
+        <label>HuggingFace Token</label>
+        <input type="text" id="diar-hf-token" placeholder="hf_...">
+      </div>
+      <p style="font-size:.78rem;color:var(--muted);margin-bottom:10px;">
+        Token benötigt Lesezugriff auf
+        <a href="https://huggingface.co/pyannote/speaker-diarization-3.1" target="_blank"
+           style="color:var(--yellow);text-decoration:none;">pyannote/speaker-diarization-3.1</a>.
+      </p>
+    </section>
   </main>
   <div class="toast" id="toast"></div>
   <script src="/settings.js"></script>