feat(anime): queue episodes to play next across anime

- Episode rows gain Play/Queue actions (and matching context-menu items); queued rows show their place in line, with a queue count and Clear queue in the episode header
- Queue lives in the main process (`anime-browser-queue.ts`) so it survives the browser window closing and advances on mpv's end-file even when nobody is watching; streams resolve at play time so a signed URL cannot expire while queued
- Holds mpv's keep-open off while the queue waits and restores it once empty; queueing with nothing playing just plays immediately
- Adds anime-browser-queue and episode-queue unit tests, IPC channels/contracts, and doc updates

src/animeui/animeui.ts

@@ -405,8 +405,15 @@ loadMoreButton.addEventListener('click', () => void loadNextPage());
 
 api.onBridgeState(renderBridgeState);
 
+// The queue changes without this window asking: it advances by itself when an
+// episode ends, whether or not anyone is looking at the browser.
+api.onQueueState((state) => detailPanel.setQueue(state));
+
 void (async () => {
   renderBridgeState({ stage: 'idle', progress: null, message: null });
+  // A queue survives the window being closed and reopened, so start from what
+  // the main process already holds rather than from empty.
+  void api.getQueue().then((state) => detailPanel.setQueue(state));
   try {
     const state = await api.ensureBridge();
     renderBridgeState(state);