feat(launcher): add post-playback history menu with previous episode

- After a history episode ends or mpv closes, show a menu to play the previous/next episode, rewatch, browse, or quit, looping back to the same series
- Add findPreviousEpisode to step backward within and across season directories
- Add quit option to the initial history series-action menu
- Dedupe signal-handler registration in playback-command so repeated history sessions don't stack SIGINT/SIGTERM handlers
- Update README and docs-site for the new history flow

launcher/commands/playback-command.test.ts

@@ -5,7 +5,7 @@ import fs from 'node:fs';
 import os from 'node:os';
 import path from 'node:path';
 import type { LauncherCommandContext } from './context.js';
-import { runPlaybackCommandWithDeps } from './playback-command.js';
+import { registerCleanup, runPlaybackCommandWithDeps } from './playback-command.js';
 import { state } from '../mpv.js';
 
 function createContext(): LauncherCommandContext {
@@ -103,6 +103,20 @@ function createContext(): LauncherCommandContext {
   };
 }
 
+test('playback cleanup signal handlers are registered once across repeated sessions', () => {
+  assert.equal(typeof registerCleanup, 'function', 'cleanup registration is not exported');
+  const context = createContext();
+  const registeredSignals: NodeJS.Signals[] = [];
+  context.processAdapter.onSignal = (signal) => {
+    registeredSignals.push(signal);
+  };
+
+  registerCleanup(context);
+  registerCleanup(context);
+
+  assert.deepEqual(registeredSignals, ['SIGINT', 'SIGTERM']);
+});
+
 test('youtube playback launches overlay with app-owned youtube flow args', async () => {
   const calls: string[] = [];
   const context = createContext();