feat(playlists): select rows with a box, Shift for a range, x and Esc

A box first in every row and one over the column for the rows shown, the
selected row tinted. A filter keeps what it hides selected; another list
starts over; a row that left the list drops out. Watch later has none.

A box no longer counts as a typing target: a click on one left the focus
there and the page's keys went dead.

frontend/src/lib/tableColumns.ts

@@ -24,6 +24,8 @@ type ColumnFilter<T> =
 export interface Column<T> {
   key: string;
   header: string;
+  // A header that is a control of its own (the box that selects every row), in place of `header`.
+  headerCell?: ReactNode;
   render: (row: T) => ReactNode;
   align?: "left" | "right" | "center";
   width?: string;