demo: fix header (no sticky), make Display None interactive, add opacity note

examples/demo/app.gltm

@@ -18,6 +18,7 @@
 @global $alert   = 70.0
 @global $nodes   = ["Alpha", "Bravo", "Charlie", "Delta"]
 @global $notif   = true
+@global $show    = true
 
 @singleton AppInfo {
     build = "2026-07-22",
@@ -191,6 +192,7 @@ Window(title=$title) {
                     Panel(class="op-85")  { Text "85%" }
                     Panel(class="op-100") { Text "100%" }
                 }
+                Text(class="gauge-label") "* Opacity depends on Iced widget support — works on containers, not on text"
             }
 
             // Panel: Scroll + Sticky
@@ -221,13 +223,18 @@ Window(title=$title) {
                 }
             }
 
-            // Panel: Display None
+            // Panel: Display None (interactive toggle via @if)
             Panel(class="card") {
                 Header "Display: None"
                 Panel(class="demo-row") {
-                    Panel(class="vis")   { Text "Visible"   }
-                    Panel(class="hid")   { Text "Hidden"    }
-                    Panel(class="vis")   { Text "Still Here" }
+                    Panel(class="vis") { Text "Always" }
+                    @if $show {
+                        Panel(class="vis") { Text "Toggled" }
+                    }
+                    Panel(class="vis") { Text "Always" }
+                }
+                Button(label="Toggle Center Element", class="btn-outline") {
+                    @on click { !rhei: { show = !show; } }
                 }
             }