fix(playlists): what the first review of bulk actions found

An Esc during a pointer drag is spent on the drag and leaves the selection.
A wheel over the rail re-reads the target each frame, so a drop lands on the
row under the pointer. After a bulk removal the focus goes to the next row
left, and to the list's menu when the list is emptied.

remove-bulk names the videos that went, and its Undo brings back only those.
An Undo of a move keeps each row (and when it was saved), leaves out a video
that moved on from the target, closes up its place, and says how many came
back. Delete with every tick hidden removes nothing and says why. A list gone
before the lock answers 404, and a body without its videos 422.

One removal path now: the single row goes through the bulk one, the same
toast and undo, which also retires the add-at-position route. railDrop reuses
the store and scrollParent, the drag image the overlay and the z-index token,
and the sensors keep their number. The count is read out from a region that
outlives the band. Tests fill the gaps the review named, each seen failing.

backend/tests/test_reader_service.py

@@ -210,9 +210,8 @@ def test_an_add_is_idempotent_and_lands_where_asked(db, make_user):
     pl = service.create_playlist(db, user, "L")
     assert service.add_item(db, pl, "a", mark_dirty=False) is True
     assert service.add_item(db, pl, "a", mark_dirty=False) is False
-    service.add_to_playlist(db, pl, "b", None)
-    service.add_to_playlist(db, pl, "c", 0)
-    service.add_to_playlist(db, pl, "a", 2)  # already there: stays put
+    assert service.add_item(db, pl, "b", mark_dirty=True) is True
+    assert service.restore_items(db, pl, ["c", "a"], [0, 2]) == 1  # "a" is there: it stays put
     assert _uncommitted(db)
     db.commit()
     assert _ids(db, pl) == ["c", "a", "b"]