Show empty asset groups on request, and add by the heading (#361)

The asset panel only listed the categories and types that hold assets.
To make the first asset of a kind you had to know its category in the
`+` menu. A switch now lists all of them, and a right-click on a
heading offers just what sits under it.

- General settings has "Show empty asset categories and types", off by
  default and kept in this browser. Applying it refreshes the panel at
  once through a new `onGeneralSettings` listener.
- `groupAssets(assets, empty)` gives every category and type when
  asked. The empty ones stand faint with a count of 0, and an empty
  type has no chevron, since there is nothing to unfold.
- `ListItem.faint` draws a row's label, hint and icons at half opacity,
  like a disabled widget.
- A right-click on a category heading offers the types of that
  category without a submenu, on a type heading only that type. The
  empty space below the rows still opens the whole `+` menu.
  `ListView.onListMenu` passes the heading's id for this.
- Every menu of new assets says "Create asset" above its entries, so a
  click on a type reads as making one. `MenuOptions.title` draws that
  muted line. It is not an entry and matches no search.

Tests:
- Unit asset-groups and view-tween: 23 green, 2 new. Counter-test
  with the empty flag ignored: the new groups test fails.
- e2e 04 (melektaus-atx): ALL PASSED, 67 checks, 4 new and 1
  rewritten. Counter-test with the heading id dropped: the 3 menu
  checks fail. The title is checked on the `+` button and on a type
  heading (2 checks extended), the rerun passed with 67 checks.
- Gate (melektaus-atx): CHECK EXIT 0, 272 files, 4212 tests. It ran
  before the menu title. Afterwards only the menu and asset-groups
  units, typecheck and lint ran, all clean.

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

tests/editor/asset-groups.test.ts

@@ -63,6 +63,18 @@ describe('groupAssets (#176)', () => {
     expect(groupsOf([second, first])[0]!.assets).toEqual([second, first]);
   });
 
+  it('lists every category and type when asked to, the empty ones with no assets (#361)', () => {
+    const categories = groupAssets([asset('sprite', 'Hero')], true);
+    expect(categories.map((c) => c.category)).toEqual([...ASSET_CATEGORIES]);
+    const groups = categories.flatMap((c) => c.groups);
+    expect(groups.map((g) => g.type).sort()).toEqual([...ASSET_TYPES].sort());
+    expect(groups.filter((g) => g.assets.length > 0).map((g) => g.type)).toEqual(['sprite']);
+    for (const { category, groups } of categories)
+      for (const { type } of groups) expect(ASSET_GROUPS[type].category).toBe(category);
+    expect(groupAssets([], true).flatMap((c) => c.groups)).toHaveLength(ASSET_TYPES.length);
+    expect(groupAssets([])).toEqual([]);
+  });
+
   it('leaves the project list alone', () => {
     const assets = [asset('sprite', 'B'), asset('sprite', 'A')];
     groupAssets(assets);