feat: play the next video automatically when one ends

A toggle under Playback in Settings, off by default and remembered.

Verified offline with forced offline mode on: a downloaded video ran to
its end and the next one started by itself, 6/51 to 7/51, with no
network.

It follows the same list the Next button does, which already steps to
the next *playable* video — offline that is the next download, so it
plays through what is on the Mac one after another. Online it will
stream the next video, and the Settings text says so: a toggle that
quietly does nothing outside one hidden condition is how the subtitle
preference went wrong three times.

The last video in the list simply stops; there is nowhere to go and
Next is already absent there.

src/components/Player.tsx

@@ -27,6 +27,8 @@ interface Props {
   onSubStyle: (s: SubStyle) => void;
   /** Leaves the player for this video's channel. */
   onOpenChannel: () => void;
+  /** Roll straight on to the next video when this one ends. */
+  autoplayNext: boolean;
   /** Persists a subtitle choice made from the transport bar. */
   onSubLang: (l: string) => void;
   /** Position in the current feed, for the "3 of 180" readout. */
@@ -122,7 +124,7 @@ function placeCues(vtt: string, line: number | null): string {
 
 export default function Player({
   item, path, onClose, onDelete, onPrev, onNext, onDownload, downloading,
-  maxHeight, subLang, onSubLang, subStyle, onSubStyle, onOpenChannel,
+  maxHeight, subLang, onSubLang, subStyle, onSubStyle, onOpenChannel, autoplayNext,
   index, total, titleBarInset,
 }: Props) {
   const streaming = path === null;
@@ -457,6 +459,12 @@ export default function Player({
             onTimeUpdate={onTimeUpdate}
             onLoadedMetadata={onLoadedMetadata}
             onPause={persist}
+            onEnded={() => {
+              persist();
+              // Only where there is somewhere to go: the last video in the
+              // list simply stops.
+              if (autoplayNext) onNext?.();
+            }}
             onLoadStart={() => setBuffering(true)}
             onWaiting={() => setBuffering(true)}
             onStalled={() => setBuffering(true)}