holder: fix scad editor panel hidden bug + smaller default height

The CSS rule `.scad-panel { display: flex }` was overriding the HTML
`hidden` attribute (CSS specificity beats the user-agent rule on the
attribute). Effect: the Close button set hidden=true but the panel
stayed visible, and the panel was visible on first load too.

- Explicit `.scad-panel[hidden] { display: none }` rule restores the
  intended behaviour: Close hides, initial load is hidden.
- Default height 45% -> 30%, with min-height 180px so the editor
  isn't cramped even on small viewports.

static/holder.css

@@ -178,12 +178,19 @@
 .scad-panel {
   position: absolute;
   left: 0; right: 0; bottom: 0;
-  height: 45%;
+  height: 30%;
+  min-height: 180px;
   background: var(--bg, #11141a);
   border-top: 1px solid var(--border, #2a2f3a);
   display: flex;
   flex-direction: column;
   z-index: 2;
+  overflow: hidden;
+}
+/* The [hidden] attribute alone is overridden by display:flex above —
+ * make the hide explicit so the Close button actually closes the panel. */
+.scad-panel[hidden] {
+  display: none;
 }
 .scad-panel-bar {
   display: flex;