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/NativeControls.tsx

@@ -1,5 +1,5 @@
 import clsx from "clsx";
-import type { InputHTMLAttributes, ReactNode, Ref } from "react";
+import type { InputHTMLAttributes, ReactNode, RefObject } from "react";
 
 // A6 (docs/DESIGN.md): checkbox, slider and colour well are the BROWSER's controls, tinted
 // with the chosen colour (the accent; the media amber on the player chrome) — keyboard, screen
@@ -10,18 +10,30 @@ import type { InputHTMLAttributes, ReactNode, Ref } from "react";
 
 type Native = Omit<InputHTMLAttributes<HTMLInputElement>, "type" | "onChange">;
 
-/** The bare checkbox: inside your own `<label>`, or with an `aria-label`. Prefer `CheckRow`. */
+/** The bare checkbox: inside your own `<label>`, or with an `aria-label`. Prefer `CheckRow`.
+ *  `onChange` also says whether Shift was held, for a range. `mixed` is the third state, some of what
+ *  it covers: the DOM has no attribute for it, only the property. */
 export function Checkbox({
   onChange,
   className,
   ref,
+  mixed = false,
   ...rest
-}: Native & { onChange?: (checked: boolean) => void; ref?: Ref<HTMLInputElement> }) {
+}: Native & {
+  onChange?: (checked: boolean, shift: boolean) => void;
+  ref?: RefObject<HTMLInputElement | null>;
+  mixed?: boolean;
+}) {
   return (
     <input
-      ref={ref}
+      ref={(el) => {
+        if (ref) ref.current = el;
+        if (el) el.indeterminate = mixed;
+      }}
       type="checkbox"
-      onChange={(e) => onChange?.(e.target.checked)}
+      onChange={(e) =>
+        onChange?.(e.target.checked, e.nativeEvent instanceof MouseEvent && e.nativeEvent.shiftKey)
+      }
       className={clsx(
         "size-4 shrink-0 cursor-pointer accent-chosen disabled:cursor-not-allowed",
         className,
@@ -54,7 +66,7 @@ export function CheckRow({
         className,
       )}
     >
-      <Checkbox checked={checked} onChange={onChange} disabled={disabled} />
+      <Checkbox checked={checked} onChange={(on) => onChange(on)} disabled={disabled} />
       {children}
     </label>
   );