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>

src/main.rs

@@ -0,0 +1,20 @@
+#![cfg_attr(not(debug_assertions), windows_subsystem = "windows")]
+
+mod app;
+mod downloader;
+mod library;
+
+fn main() -> eframe::Result<()> {
+    let native_options = eframe::NativeOptions {
+        viewport: eframe::egui::ViewportBuilder::default()
+            .with_inner_size([1280.0, 820.0])
+            .with_min_inner_size([800.0, 500.0])
+            .with_title("YouTube Backup"),
+        ..Default::default()
+    };
+    eframe::run_native(
+        "youtube-backup",
+        native_options,
+        Box::new(|cc| Ok(Box::new(app::App::new(cc)))),
+    )
+}