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/main.ts

@@ -3370,6 +3370,12 @@ const animeBrowserRuntime = createAnimeBrowserRuntime({
       window.webContents.send(IPC_CHANNELS.event.animeBrowserSearchUpdate, update);
     }
   },
+  onQueueState: (state) => {
+    const window = appState.animeBrowserWindow;
+    if (window && !window.isDestroyed()) {
+      window.webContents.send(IPC_CHANNELS.event.animeBrowserQueueState, state);
+    }
+  },
   log: (message) => logger.info(message),
 });