feat: Library bulk download + list virtualization for large channels

Add a source-wide "Select all" across all playlist groups, and make one
click queue every indexed video (the previous 100-per-click cap is gone).
Bulk download skips items that are already downloaded or in flight, so it
can't enqueue duplicates, and it includes failed/canceled ones so it also
retries them.

Make the UI hold up at channel scale (1000s of videos): virtualize both
the Downloads queue and the Library item list with @tanstack/react-virtual
so only on-screen rows hit the DOM, memoize the queue row so they don't all
reconcile on every progress tick, and batch the enqueue so queuing a whole
channel stays O(n) instead of O(n^2).

- VirtualList: reusable windowed list that owns its own scroll container
- DownloadsView: virtualized, flex-filled queue
- LibraryView: groups flattened to rows; virtualized panel above 100 rows,
  inline below so small sources are unchanged
- QueueItem: React.memo
- downloads store: addMany (one state update + one pump); sources:
  enqueue the whole selection via addMany, no cap

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

package.json

@@ -25,6 +25,7 @@
     "@electron-toolkit/utils": "^4.0.0",
     "@fluentui/react-components": "^9.74.1",
     "@fluentui/react-icons": "^2.0.330",
+    "@tanstack/react-virtual": "^3.14.3",
     "electron-store": "^11.0.2",
     "react": "^19.2.7",
     "react-dom": "^19.2.7",