fix: always show user badge for logout access

When username is empty (e.g. email-registered OIDC users with no display
name), the badge was hidden entirely, making logout inaccessible. Always
render the badge with fallback text "Account".

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

ui/src/components/LayoutShell.tsx

@@ -1062,7 +1062,7 @@ function LayoutContent() {
             <NotificationBell />
           </div>
         }
-        user={username ? { name: username } : undefined}
+        user={{ name: username || 'Account' }}
         userMenuItems={userMenuItems}
         onLogout={handleLogout}
         onNavigate={navigate}