fix(downloads): harden queue controls and resumable replacement

Prevent queue action clicks from triggering row double-clicks, preserve aria2/yt-dlp resumable sidecars during duplicate replacement, and make aria2 require resume instead of silently restarting.

Add a persisted, accessible Folders collapse control with reduced-motion animation and regression coverage for replacement sidecar handling.

Fixes #11

Fixes #12

Closes #13

Refs #14

src/components/DownloadItem.tsx

@@ -177,6 +177,7 @@ export const DownloadItem = React.memo<DownloadItemProps>(({
         
         <div
           className="hidden group-hover:flex items-center justify-end gap-0.5 w-full ml-auto"
+          onClick={(e) => e.stopPropagation()}
           onDoubleClick={(e) => e.stopPropagation()}
         >
           {(download.status === 'queued' || download.status === 'staged') && queueIndex !== -1 && (