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>

README.md

@@ -43,7 +43,7 @@ npm run build        # writes dist/ and regenerates index.html; the folder is wh
 The browser suite and the three example games need a machine with a GPU:
 
 ```bash
-node tools/seed-e2e.mjs && node tools/e2e.mjs      # the editor, 1452 checks (GM_STEPS=20 for one step)
+node tools/seed-e2e.mjs && node tools/e2e.mjs      # the editor, 1455 checks (GM_STEPS=20 for one step)
                                                    #   every tool signs in first – see the Accounts page
 node tools/seed-jumprun.mjs && node tools/smoke-jumprun.mjs
 node tools/seed-tetris.mjs && node tools/smoke-tetris.mjs