feat(youtube): add YouTube browser window and Whisper subtitle source

- Add `subminer youtube`/`yt` and tray "Browse YouTube" window that plays or queues picked videos in mpv
- Add `youtube.subtitleSource` (`youtube` | `whisper`) to transcribe YouTube audio via subtitle generation
- Allow the subtitle generation modal on YouTube videos
- Pick the real Japanese track over machine-translated ones and fall back to yt-dlp on HTTP 429
- Remove legacy launcher youtubeSubgen/whisper/AI args and `--youtube-mode`
- Fix EPIPE crash when the launching terminal is closed with Ctrl+C
- Thin media timing review handles with orange original-boundary bars
- Update docs and change fragments

src/renderer/modals/media-timing-review.ts

@@ -449,8 +449,8 @@ export function createMediaTimingReviewModal(
   }
 
   /**
-   * Moves an untouched clip end back to where the line's dialogue ends. The Line end
-   * rail keeps marking the subtitle timing, and Reset restores it.
+   * Moves an untouched clip end back to where the line's dialogue ends. The orange
+   * end bar keeps marking the subtitle timing, and Reset restores it.
    */
   function trimTrailingSilence(peaks: readonly number[]): void {
     if (!payload || !trailingTrimPending || previewPlaying || previewRequest.isInFlight()) {

src/renderer/modals/subtitle-generation.ts

@@ -88,7 +88,7 @@ export function createSubtitleGenerationModal(
     const vad = snapshot ? describeGenerationVad(snapshot.vad) : null;
     const tools = snapshot ? describeGenerationTools(snapshot.tools) : null;
     const readyMessage = !snapshot?.mediaPath
-      ? 'Open local media to generate subtitles.'
+      ? 'Open a local file or a YouTube video to generate subtitles.'
       : !tools?.ready
         ? 'Install the missing tools or set their paths in Settings, then click Check again.'
         : !model?.ready
@@ -96,7 +96,8 @@ export function createSubtitleGenerationModal(
           : !vad?.ready
             ? 'Download the speech detection model or uncheck Focus on spoken dialogue.'
             : 'Ready when you are.';
-    dom.media.textContent = snapshot?.mediaPath ?? 'Open a local media file in the player first.';
+    dom.media.textContent =
+      snapshot?.mediaPath ?? 'Open a local file or a YouTube video in the player first.';
     dom.tools.textContent = tools?.text ?? 'Checking local tools...';
     dom.model.textContent = model?.text ?? 'Checking local models...';
     dom.modelPicker.classList.toggle('hidden', !snapshot || Boolean(snapshot.externalModelPath));