feat: add mark-as-watched keybinding and Yomitan lookup tracking

Add configurable keybinding to mark the current video as watched with
IPC plumbing between renderer and main process. Add event listener
infrastructure for tracking Yomitan dictionary lookups per session.

src/renderer/yomitan-popup.test.ts

@@ -0,0 +1,18 @@
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import { YOMITAN_LOOKUP_EVENT, registerYomitanLookupListener } from './yomitan-popup.js';
+
+test('registerYomitanLookupListener forwards the SubMiner Yomitan lookup event', () => {
+  const target = new EventTarget();
+  const calls: string[] = [];
+
+  const dispose = registerYomitanLookupListener(target, () => {
+    calls.push('lookup');
+  });
+
+  target.dispatchEvent(new CustomEvent(YOMITAN_LOOKUP_EVENT));
+  dispose();
+  target.dispatchEvent(new CustomEvent(YOMITAN_LOOKUP_EVENT));
+
+  assert.deepEqual(calls, ['lookup']);
+});