fix: await logout before navigating, prevent cancelled fetch

- Logout is now async — awaits sidecar /auth/logout before clearing tokens
- confirmSignOut awaits logout() before navigate('/login')
- 5 second timeout on logout fetch to prevent indefinite hang
- Added console.warn on logout failure

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

src/components/layout/sidebar.tsx

@@ -132,9 +132,9 @@ export const Sidebar = ({ activeUrl = "/" }: SidebarProps) => {
         setLogoutOpen(true);
     };
 
-    const confirmSignOut = () => {
+    const confirmSignOut = async () => {
         setLogoutOpen(false);
-        logout();
+        await logout();
         navigate('/login');
     };