refactor(download manager): Added Downloadable trait and replaced references to GameDownloadAgent

Signed-off-by: quexeky <git@quexeky.dev>

desktop/src-tauri/src/download_manager/progress_object.rs

@@ -4,7 +4,7 @@ use std::{
         mpsc::Sender,
         Arc, Mutex, RwLock,
     },
-    time::{Duration, Instant},
+    time::Instant,
 };
 
 use log::info;