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.test.ts

@@ -0,0 +1,40 @@
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import { describeQueuePosition, queueKey, queuePositions } from './episode-queue';
+import type { AnimeBrowserQueueEntry } from '../types/anime-browser';
+
+function entry(overrides: Partial<AnimeBrowserQueueEntry> = {}): AnimeBrowserQueueEntry {
+  return {
+    sourceId: 'source',
+    animeUrl: '/anime',
+    animeTitle: 'Anime',
+    episodeUrl: '/episode-1',
+    episodeName: 'Episode 1',
+    episodeNumber: 1,
+    ...overrides,
+  };
+}
+
+test('positions count across the whole queue, not within one anime', () => {
+  const positions = queuePositions([
+    entry(),
+    entry({ animeUrl: '/other', episodeUrl: '/other-1' }),
+    entry({ episodeUrl: '/episode-2' }),
+  ]);
+
+  assert.equal(positions.get(queueKey('source', '/episode-1')), 1);
+  assert.equal(positions.get(queueKey('source', '/other-1')), 2);
+  assert.equal(positions.get(queueKey('source', '/episode-2')), 3);
+});
+
+test('the same episode url on two sources keeps two positions', () => {
+  const positions = queuePositions([entry(), entry({ sourceId: 'other' })]);
+
+  assert.equal(positions.get(queueKey('source', '/episode-1')), 1);
+  assert.equal(positions.get(queueKey('other', '/episode-1')), 2);
+});
+
+test('the episode about to play is named rather than numbered', () => {
+  assert.equal(describeQueuePosition(1), 'next up');
+  assert.equal(describeQueuePosition(4), '#4 in queue');
+});