fix(queue-panel): set flow layout _itemSize to match actual track item height

The previous fix (fixed CSS height on .track-item) was insufficient because
lit-virtualizer's flow layout defaults to estimating items at 100px tall.
With 20k items, the difference between 100px estimate and 49px actual creates
a ~1M px scroll height that collapses as items get measured, triggering
scroll error corrections (programmatic scrollTo calls) that fight the native
scrollbar during drag.

Setting _itemSize to { height: 49 } via the flow() config ensures the initial
scroll size estimate matches reality, eliminating the scroll error corrections
that caused the scrollbar thumb to desync from the mouse when dragging down.

frontend/src/components/queue-panel/queue-panel.ts

@@ -127,7 +127,15 @@ export class QueuePanel
     };
 
     private panelWidth = DEFAULT_WIDTH;
-    private flowLayout = flow();
+    // _itemSize is an internal property applied via Object.assign in BaseLayout's
+    // config setter. Setting it to match the actual fixed .track-item height (49px)
+    // prevents lit-virtualizer's scroll error correction from fighting the native
+    // scrollbar during drag on large lists (20k+ items). Without this, the default
+    // estimate of 100px causes massive scroll height recalculation as items get
+    // measured, which calls scrollTo() and desynchronizes the scrollbar thumb.
+    private flowLayout = flow({
+        _itemSize: { width: 100, height: 49 },
+    } as Parameters<typeof flow>[0]);
 
     /**
      * Track the last currentIndex so we only auto-scroll