feat: AI sidebar mobile full-width overlay with back button

- Mobile: expanded sidebar is fixed inset-0 full-screen overlay with back button
- Mobile: collapsed icon strip hidden (no space on mobile)
- Desktop: unchanged resizable panel + collapsed icon strip

frontend/src/components/layout/AISidebar.tsx

@@ -27,11 +27,11 @@ export function AISidebar() {
     })();
   }, []);
 
-  // Collapsed: narrow icon strip
+  // Collapsed: narrow icon strip (desktop only, hidden on mobile)
   if (aiSidebarCollapsed) {
     return (
       <div
-        className="flex-shrink-0 w-12 flex flex-col items-center border-l border-secondary-200 bg-white py-3"
+        className="hidden md:flex flex-shrink-0 w-12 flex-col items-center border-l border-secondary-200 bg-white py-3"
         data-testid="ai-sidebar-collapsed"
       >
         <button
@@ -48,13 +48,47 @@ export function AISidebar() {
     );
   }
 
-  // Expanded: full chat panel with resize handle
+  // Expanded: Mobile = full width overlay, Desktop = resizable panel
   return (
+    <>
+      {/* Mobile: full width overlay with back button */}
+      <div
+        className="md:hidden fixed inset-0 z-50 bg-white flex flex-col"
+        data-testid="ai-sidebar-mobile"
+      >
+        <div className="h-14 flex items-center gap-2 px-3 border-b border-secondary-200">
+          <button
+            onClick={toggleAISidebar}
+            className="inline-flex items-center gap-1 px-2 py-1 rounded text-sm text-secondary-700 hover:bg-secondary-100 min-h-touch"
+            aria-label="Zurück"
+            data-testid="ai-sidebar-back"
+          >
+            <svg className="w-5 h-5" fill="none" viewBox="0 0 24 24" stroke="currentColor" aria-hidden="true">
+              <path strokeLinecap="round" strokeLinejoin="round" strokeWidth={2} d="M15 19l-7-7 7-7" />
+            </svg>
+            <span className="text-sm font-medium">Zurück</span>
+          </button>
+          <span className="text-sm font-medium text-secondary-700 ml-2">KI Assistent</span>
+        </div>
+        <div className="flex-1 overflow-hidden">
+          {loading ? (
+            <div className="flex items-center justify-center h-full text-sm text-secondary-400">Laden...</div>
+          ) : sessionId ? (
+            <ChatWindow sessionId={sessionId} compact className="h-full" />
+          ) : (
+            <div className="flex items-center justify-center h-full text-sm text-red-500">
+              Session konnte nicht erstellt werden
+            </div>
+          )}
+        </div>
+      </div>
+
+      {/* Desktop: resizable panel */}
       <ResizablePanel
         initialWidth={320}
         minWidth={240}
         maxWidth={600}
-      className="border-l border-secondary-200 bg-white"
+        className="hidden md:flex border-l border-secondary-200 bg-white"
         data-testid="ai-sidebar-pane"
       >
         <div className="h-full flex flex-col">
@@ -84,5 +118,6 @@ export function AISidebar() {
           </div>
         </div>
       </ResizablePanel>
+    </>
   );
 }