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/components/ui/NavGrid.tsx

@@ -1,5 +1,5 @@
 import { useRef, type ReactNode } from "react";
-import { useListNav } from "../../lib/listNav";
+import { useListNav, type NavSelection } from "../../lib/listNav";
 
 // A plain list of cards or rows the keyboard can walk — `j`/`k`, the arrows, Enter (lib/listNav) — for
 // the lists that mount every item (the Plex title, season and episode grids, the download lists, a
@@ -11,16 +11,19 @@ export default function NavGrid({
   as: Tag = "div",
   className,
   onDelete,
+  selection,
   children,
 }: {
   as?: "div" | "tbody";
   className?: string;
   /** The Delete key takes the focused item out (lib/listNav): its index among the items. */
   onDelete?: (index: number) => void;
+  /** `x` selects the focused item, Esc clears (lib/listNav). */
+  selection?: NavSelection;
   children: ReactNode;
 }) {
   const ref = useRef<HTMLDivElement & HTMLTableSectionElement>(null);
-  useListNav(ref, { enabled: true, onDelete });
+  useListNav(ref, { enabled: true, onDelete, selection });
   return (
     <Tag ref={ref} className={className}>
       {children}