Assistent: Layout-Fix, Auth-Fix, Session-Reaktivierung

- CSS: Feste Hoehe, overflow hidden, Input-Bar immer sichtbar
- Auth: Prueft username UND displayName gegen E-Mail und Name
- Beendete Sessions werden automatisch reaktiviert beim Senden

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/index.html

@@ -714,15 +714,6 @@
               <span id="assistant-status-badge" class="assistant-status-badge"></span>
             </div>
             <div id="assistant-messages" class="assistant-messages"></div>
-            <div class="assistant-input-bar">
-              <textarea id="assistant-input" class="assistant-input" placeholder="Nachricht eingeben..." rows="1"></textarea>
-              <button id="btn-send-message" class="btn btn-primary assistant-send-btn" title="Senden">
-                <svg viewBox="0 0 24 24" width="20" height="20" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round">
-                  <line x1="22" y1="2" x2="11" y2="13"/>
-                  <polygon points="22 2 15 22 11 13 2 9 22 2"/>
-                </svg>
-              </button>
-            </div>
             <div id="assistant-empty" class="assistant-empty">
               <div class="assistant-empty-icon">
                 <svg viewBox="0 0 24 24" width="64" height="64" fill="none" stroke="currentColor" stroke-width="1.5">
@@ -732,6 +723,15 @@
               <h3>Claude Assistent</h3>
               <p>Starte eine neue Session oder waehle eine bestehende aus.</p>
             </div>
+            <div class="assistant-input-bar">
+              <textarea id="assistant-input" class="assistant-input" placeholder="Nachricht eingeben..." rows="1"></textarea>
+              <button id="btn-send-message" class="btn btn-primary assistant-send-btn" title="Senden">
+                <svg viewBox="0 0 24 24" width="20" height="20" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round">
+                  <line x1="22" y1="2" x2="11" y2="13"/>
+                  <polygon points="22 2 15 22 11 13 2 9 22 2"/>
+                </svg>
+              </button>
+            </div>
           </main>
         </div>
       </div>