feat(youtube): add YouTube browser window and Whisper subtitle source (#273)

changes/closed-pipe-crash.md

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+type: fixed
+area: app
+
+- SubMiner no longer crashes with "write EPIPE" when the terminal command that started it is stopped with Ctrl+C.

changes/youtube-browser-window.md

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+type: added
+area: youtube
+
+- Added a YouTube browser window (`subminer youtube`/`subminer yt`, or tray **Browse YouTube**) that keeps your YouTube login across restarts.
+- Clicking a video plays it in mpv instead of the page; middle-click, Shift/Ctrl-click, or the right-click menu queues it in mpv's playlist.
+- Queued YouTube videos get the same Japanese subtitle setup when mpv reaches them.
+- Closing the YouTube window quits a SubMiner started by `subminer youtube` (after mpv closes, if a video is still playing).

changes/youtube-translated-caption-429.md

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+type: fixed
+area: youtube
+
+- YouTube Japanese subtitles no longer fail with "HTTP 429" on videos where YouTube also lists a machine-translated Japanese track; SubMiner now picks the real track and falls back to yt-dlp if a direct subtitle download is refused.

changes/youtube-whisper-subtitles.md

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+type: added
+area: youtube
+
+- Added `youtube.subtitleSource`: set it to `whisper` to transcribe YouTube videos with Whisper instead of downloading YouTube's captions. The default stays `youtube`.
+- Whisper mode keeps the video paused while it transcribes, with progress in the subtitle generation modal; close the modal to keep watching, or cancel to continue without subtitles.
+- The subtitle generation modal (`Ctrl+Shift+G`) now also works on YouTube videos, whatever `youtube.subtitleSource` is set to.
+- Whisper mode downloads a small audio-only stream and deletes it once generation ends, or when you switch videos, close mpv, or quit.