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>

Cargo.toml

@@ -0,0 +1,12 @@
+[package]
+name = "youtube-backup"
+version = "0.1.0"
+edition = "2021"
+description = "A small yt-dlp front-end: browse downloaded channels and queue new downloads"
+
+[dependencies]
+eframe = "0.29"
+image = { version = "0.25", default-features = false, features = ["webp", "jpeg", "png"] }
+
+[profile.release]
+opt-level = 2