Open the add menu with a right-click in the asset panel (#244)

New assets could only be added through the + button. A right-click
on a category heading or on the empty space below the rows now opens
the same menu at the pointer.

- ListView gets the callback onListMenu. It fires on a right-click
  on empty space or on a heading. Only the asset panel has headings,
  so other lists are not affected.
- AssetPanel opens its + menu from there.
- A right-click on a heading does not fold or unfold the category.
  A right-click on empty space clears the selection like a left-click.
- A right-click on an asset still opens the menu of that asset.
- The empty list now says "No assets - press + or right-click".

Tests:
- ./check.sh: 178 files, 2573 tests, CHECK EXIT 0
- e2e 1-4: ALL PASSED, 134 checks
- 3 new checks in e2e 04 with real right-clicks: menu on the
  "Scenes" heading (21 types, heading stays folded), menu on empty
  space, and "Behavior" from that menu creates one, undone again.
- Counter-probe: without the menu both right-click checks fail.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/editor/panels/AssetPanel.ts

@@ -80,10 +80,11 @@ export class AssetPanel extends Panel {
     add.onClick = () => new Menu(this.addMenuEntries()).openBelow(add);
     this.header.add(add);
 
-    this.list.emptyText = 'No assets – press +';
+    this.list.emptyText = 'No assets – press + or right-click';
     this.list.onSelect = (id) => state.select(id ? { kind: 'asset', assetId: id } : null);
     this.list.onActivate = (id) => this.activate(id);
     this.list.onContextMenu = (id, x, y) => this.openContextMenu(id, x, y);
+    this.list.onListMenu = (x, y) => new Menu(this.addMenuEntries()).open(x, y);
     this.list.onDragOut = (id) => this.startPlacementDrag(id);
     this.list.onToggle = (id, expand) => this.setFolded(id.slice(HEADING.length) as AssetType, !expand);
     this.scroll.setContent(this.list);