Improve startup dictionary sync UX and default playback keybindings

- Add default `f` fullscreen overlay binding and switch default AniSkip skip key to `Tab`
- Make character-dictionary auto-sync non-blocking at startup with tokenization gating for Yomitan mutations
- Add ordered startup OSD progress (checking/generating/updating/importing), refresh current subtitle on sync completion, and extend regression tests

src/main/runtime/current-media-tokenization-gate.test.ts

@@ -0,0 +1,42 @@
+import assert from 'node:assert/strict';
+import test from 'node:test';
+import { createCurrentMediaTokenizationGate } from './current-media-tokenization-gate';
+
+test('current media tokenization gate waits until current path is marked ready', async () => {
+  const gate = createCurrentMediaTokenizationGate();
+  gate.updateCurrentMediaPath('/tmp/video-1.mkv');
+
+  let resolved = false;
+  const waitPromise = gate.waitUntilReady('/tmp/video-1.mkv').then(() => {
+    resolved = true;
+  });
+
+  await Promise.resolve();
+  assert.equal(resolved, false);
+
+  gate.markReady('/tmp/video-1.mkv');
+  await waitPromise;
+  assert.equal(resolved, true);
+});
+
+test('current media tokenization gate resolves old waiters when media changes', async () => {
+  const gate = createCurrentMediaTokenizationGate();
+  gate.updateCurrentMediaPath('/tmp/video-1.mkv');
+
+  let resolved = false;
+  const waitPromise = gate.waitUntilReady('/tmp/video-1.mkv').then(() => {
+    resolved = true;
+  });
+
+  gate.updateCurrentMediaPath('/tmp/video-2.mkv');
+  await waitPromise;
+  assert.equal(resolved, true);
+});
+
+test('current media tokenization gate returns immediately for ready media', async () => {
+  const gate = createCurrentMediaTokenizationGate();
+  gate.updateCurrentMediaPath('/tmp/video-1.mkv');
+  gate.markReady('/tmp/video-1.mkv');
+
+  await gate.waitUntilReady('/tmp/video-1.mkv');
+});