fix: resolve UI bugs and pasting behavior

- Add global Cmd+V keyboard paste support for downloading links
- Restore striped background styling for alternate rows in download list
- Add ghost rows to seamlessly fill empty download space
- Map correctly formatted item size into the queue immediately
- Remove unused Tauri manager imports from backend

src/index.css

@@ -1026,7 +1026,8 @@
     flex-shrink: 0;
   }
 
-  .download-ghost-row.striped {
+  .download-ghost-row.striped,
+  .download-row.striped {
     background: hsl(var(--stripe-bg));
   }