Drag a whole selection and join existing groups in the hierarchy (#241)

Three follow-ups to the hierarchy groups. Dragging a row that belonged
to a multi-selection moved only that one row. The group dialog could
only create a new group. The group row drew its folder in front of the
label, unlike every other row in the hierarchy.

- Dragging a selected row moves every selected object. This holds for
  reordering, dropping into a group or between its members, dragging
  out of a group and dropping onto an object as new parent.
- The moved objects land side by side in hierarchy order, whichever
  row was grabbed. The whole move is one undo step.
- A selected child of a selected object travels with its parent, not
  on its own. Dropping the selection onto its own rows does nothing.
- Dragging a row that is not selected still moves only that row.
- hierarchyDrop.moveObjects replaces the single-object rule. One object
  is the case with one element, so the old drop tests hold unchanged.
- The group dialog has a "Group" dropdown: New group or an existing
  group. Picking a group greys out the name and turns the button into
  "Add". A group that holds all chosen objects already is not offered.
- Rename keeps using the dialog without the dropdown.
- ListRow draws an end icon at the very right and the hint left of it.
  Only the hierarchy uses iconAtEnd, so other lists do not change.
- New test hook __gm.hierarchyHint() gives the centre of a row's hint.

Not clicked in the browser: dragging a selection in the container tab
and onto an object as parent. Only the unit tests cover these.

Tests:
- ./check.sh: 178 files, 2564 tests, CHECK EXIT 0
- e2e 1 + 52: ALL PASSED, 27 checks
- 7 new unit tests in hierarchy-drop.test.ts, 4 new e2e checks in
  52i and 52j with real clicks and a real drag
- 4 counter-probes bite: drop rule ignores the selection (5 unit tests
  fail), icon column not kept free, dialog ignores the picked group,
  panel ignores the selection (only o2 lands in the group)

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

src/editor/objectGroups.ts

@@ -45,12 +45,28 @@ export function groupMenuItems(
   ];
 }
 
+/**
+ * Asks for the name of a new group – or, from a dropdown, for an existing group to put the objects
+ * into. A group all of them are in already is not offered.
+ */
 export function groupObjects(state: EditorState, source: ObjectHolder, ids: readonly string[]): void {
   const title = ids.length > 1 ? `Group ${ids.length} objects` : 'Group the object';
+  const groups = (source.groups ?? []).filter((g) => {
+    const members = groupMemberIds(source, g.id);
+    return members.length > 0 && !ids.every((id) => members.includes(id));
+  });
   GroupNameDialog.open(
-    new GroupNameDialog(title, nextGroupName(source), 'Group', (name) => {
-      state.execute(new GroupObjectsCommand(source.id, ids, name));
-    }),
+    new GroupNameDialog(
+      title,
+      nextGroupName(source),
+      'Group',
+      (name) => state.execute(new GroupObjectsCommand(source.id, ids, name)),
+      {
+        groups,
+        confirm: 'Add',
+        onJoin: (groupId) => state.execute(new SetObjectGroupCommand(source.id, ids, groupId)),
+      },
+    ),
   );
 }