SelectControl: 32px trigger scale + option-group divider in the theme picker

src/app/components/LivePreview.tsx

@@ -100,10 +100,14 @@ function ChartThemeControl() {
     // pattern); choosing it opens the builder and leaves the selection alone.
     // It closes the custom-themes block — right after the built-ins, BEFORE the
     // long preset roster — so it is visible without scrolling and sits next to
-    // the entries it manages.
+    // the entries it manages. The roster boundary itself (the divider) is set by
+    // chartThemeOptions where the order is decided, so the splice is the only
+    // index this component owns. The first preset is the divider-carrying option,
+    // so splicing right before it needs no count arithmetic.
     // TODO: action row inside a value picker (visual separation? AT surprise?)
     // parked for the batched council pass (docs/ux-second-pass.md).
-    list.splice(2 + customThemes.length, 0, {
+    const firstPreset = list.findIndex((o) => o.dividerBefore);
+    list.splice(firstPreset === -1 ? list.length : firstPreset, 0, {
       value: EDIT_THEMES,
       label: 'Edit themes…',
       detail: 'Create and manage custom themes',