Route the widget hover cursor through the theme

Button, Checkbox, the color swatch and list rows took a hard-coded
'pointer'; they now read Theme.hoverCursor, which is 'default'.

Also adds the todo note for a breadcrumb when drilling into a
sub-behavior in the graph.

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

src/ui/Theme.ts

@@ -18,6 +18,7 @@ export const Theme = {
   scrollbarWidth: 6,
   /** Horizontal indent per nesting level in tree lists. */
   treeIndent: 14,
+  hoverCursor: 'default',
   colors: {
     background: 0x1b1d22,
     panel: 0x24272e,