feat(ui): one-click display-mode buttons in the app-session bar (desktop)

Replace the display-mode dropdown with three inline buttons (right panel /
over whole app / fullscreen) so switching iframe modes is a single click.
Active mode is highlighted orange. Mobile keeps its own bar untouched.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

neode-ui/src/views/AppSession.vue

@@ -546,35 +546,14 @@ onBeforeUnmount(() => {
   opacity: 0.5;
 }
 
-/* Mode dropdown */
-.mode-option {
-  display: flex;
-  align-items: center;
-  gap: 8px;
-  width: 100%;
-  padding: 10px 14px;
-  font-size: 13px;
-  color: rgba(255, 255, 255, 0.7);
-  transition: all 0.15s ease;
-  text-align: left;
-}
-.mode-option:hover {
-  background: rgba(255, 255, 255, 0.08);
-  color: white;
-}
-.mode-option-active {
+/* Active display-mode button in the header bar */
+.app-session-btn-active {
   color: #fb923c;
-  background: rgba(251, 146, 60, 0.08);
+  background: rgba(251, 146, 60, 0.12);
 }
-
-.menu-fade-enter-active,
-.menu-fade-leave-active {
-  transition: opacity 0.15s ease, transform 0.15s ease;
-}
-.menu-fade-enter-from,
-.menu-fade-leave-to {
-  opacity: 0;
-  transform: translateY(-4px);
+.app-session-btn-active:hover {
+  color: #fb923c;
+  background: rgba(251, 146, 60, 0.18);
 }
 
 .content-fade-enter-active,