Initial commit: yt-dlp front-end with channel browser

Rust/egui desktop app:
- library.rs: scans channels/<name>/ into channels + videos by filename stem
- downloader.rs: runs yt-dlp in a background thread, streams progress to the UI
- app.rs: channel sidebar, searchable video list with thumbnails, detail/description
  panel, downloads panel, plays videos via mpv (falls back to xdg-open)

channels/ is gitignored (holds the downloaded media).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

.gitignore

@@ -0,0 +1,8 @@
+# Downloaded channel archives
+/channels/
+
+# Node
+node_modules/
+dist/
+*.log
+.DS_Store