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/episode-queue.ts

@@ -0,0 +1,28 @@
+import type { AnimeBrowserQueueEntry } from '../types/anime-browser';
+
+/**
+ * Where each queued episode sits in line, keyed by source and episode url.
+ *
+ * The position is counted across the whole queue rather than within one anime:
+ * "3rd up" has to mean the same thing on every detail page, or the number is
+ * worse than no number at all.
+ */
+export function queuePositions(entries: AnimeBrowserQueueEntry[]): Map<string, number> {
+  const positions = new Map<string, number>();
+  entries.forEach((entry, index) => {
+    const key = queueKey(entry.sourceId, entry.episodeUrl);
+    // A duplicate should never reach the renderer, but if one did, the earlier
+    // position is the one that will actually play.
+    if (!positions.has(key)) positions.set(key, index + 1);
+  });
+  return positions;
+}
+
+export function queueKey(sourceId: string, episodeUrl: string): string {
+  return `${sourceId} ${episodeUrl}`;
+}
+
+/** "Next up" reads better than "#1" for the episode about to play. */
+export function describeQueuePosition(position: number): string {
+  return position === 1 ? 'next up' : `#${position} in queue`;
+}