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/css/assistant.css

@@ -5,10 +5,18 @@
  */
 
 /* Layout */
+.view.view-assistant {
+  height: calc(100vh - var(--header-height) - 52px);
+  overflow: hidden;
+  display: flex;
+  flex-direction: column;
+}
+
 .assistant-layout {
   display: grid;
   grid-template-columns: 280px 1fr;
-  height: calc(100vh - 120px);
+  flex: 1;
+  min-height: 0;
   overflow: hidden;
 }
 
@@ -110,6 +118,7 @@
 .assistant-chat {
   display: flex;
   flex-direction: column;
+  height: 100%;
   overflow: hidden;
   position: relative;
   background: var(--bg-main);
@@ -201,6 +210,7 @@
 /* Messages Area */
 .assistant-messages {
   flex: 1;
+  min-height: 0;
   overflow-y: auto;
   padding: 20px;
   display: flex;
@@ -365,10 +375,12 @@
 
 /* Empty State */
 .assistant-empty {
-  position: absolute;
-  top: 50%;
-  left: 50%;
-  transform: translate(-50%, -50%);
+  flex: 1;
+  min-height: 0;
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  justify-content: center;
   text-align: center;
   color: var(--text-muted);
   pointer-events: none;
@@ -391,12 +403,7 @@
 }
 
 /* Hide empty state when messages present */
-.assistant-messages:not(:empty) ~ .assistant-empty {
-  display: none;
-}
-
-/* Hide input bar when no session */
-.assistant-chat.no-session .assistant-input-bar {
+.assistant-messages:not(:empty) + .assistant-empty {
   display: none;
 }