Add the Rust/egui app sources

- 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 / main.rs: channel sidebar, searchable thumbnail list, detail/description
  panel, downloads panel; plays videos via mpv (falls back to xdg-open)

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

.gitignore

@@ -1,8 +1,9 @@
-# Downloaded channel archives
+# Downloaded channel archives (videos, thumbnails, descriptions)
 /channels/
 
-# Node
-node_modules/
-dist/
-*.log
+# Rust build artifacts
+/target/
+
+# Editor / OS cruft
+*.swp
 .DS_Store