feat(ui): implement multi-select and fix finder reveal

- Fix 'Show in finder' for unfinished entries to open destination folder
- Add 'Add to queue' submenu for unfinished items
- Support Shift/Cmd modifiers for multi-selection in download list
- Implement mass-action context menu for multi-selected downloads
- Enhance DeleteConfirmationModal to handle mass deletions gracefully

src/index.css

@@ -1499,6 +1499,10 @@
   .download-row:nth-child(even) {
     background: hsl(var(--stripe-bg));
   }
+  
+  .download-row.is-selected {
+    background: hsl(var(--accent-color) / 0.15) !important;
+  }
 
   .download-row:hover {
     background: hsl(var(--item-hover));