feat: add thumbnail extraction and timeline display

Wire ffmpeg frame extraction into post-download processing and render
adaptive thumbnail strips on the timeline via the asset protocol.

Co-authored-by: Cursor <cursoragent@cursor.com>

src-tauri/src/lib.rs

@@ -18,6 +18,7 @@ pub fn run() {
             video::start_download,
             media_analysis::extract_keyframes,
             media_analysis::extract_waveform,
+            media_analysis::extract_thumbnails,
         ])
         .run(tauri::generate_context!())
         .expect("error while running tauri application");