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>

src/editor/debug/uiHooks.ts

@@ -274,6 +274,8 @@ export function uiHooks({ app, state, editor }: HookContext) {
     tooltip: () => tooltips.shown,
     /** Entries of the open menu in drawn order, `null` for a separator line (#43). */
     menuRows: () => currentMenu()?.rowsDrawn() ?? null,
+    /** The muted title above the entries of the open menu, `null` without one (#361). */
+    menuTitle: () => currentMenu()?.titleDrawn() ?? null,
     /** Per entry of `menuRows`, whether an icon stands in front of it. */
     menuIcons: () => currentMenu()?.iconsDrawn() ?? null,
     /** Per entry of `menuRows`, whether it opens a submenu. With one open, `menuRows` is that submenu. */
@@ -358,6 +360,11 @@ export function uiHooks({ app, state, editor }: HookContext) {
       const dialog = editor.generalSettingsDialog();
       return dialog ? centreOf(dialog.zoomControl) : null;
     },
+    /** Where to click the switch for empty asset groups inside that dialog (#361). */
+    generalSettingsEmptyGroupsAt: () => {
+      const dialog = editor.generalSettingsDialog();
+      return dialog ? centreOf(dialog.emptyGroupsControl) : null;
+    },
     /**
      * The zoom transition in ms as kept in this browser; with `ms` it is set first. The harness sets
      * 0 on every load, so a check reading the zoom after the wheel finds the step and not the way.
@@ -391,6 +398,8 @@ export function uiHooks({ app, state, editor }: HookContext) {
     /** The asset row lighting up after a ping (#221) and how brightly, or null once it has faded. */
     assetPing: () => assets.pingState(),
     assetRowColor: (id: string) => assets.rowColor(id),
+    /** Opacity of a shown row's label, below 1 for an empty category or group (#361). */
+    assetRowAlpha: (id: string) => assets.rowAlpha(id),
     /** The asset panel's groups in order; a heading's row is `assetRow('group:<type>')`. */
     assetGroups: () => assets.groups(),
     /** The categories above the groups; a heading's row is `assetRow('category:<name>')`. */