fix(call): rAF batch flush never emptied into state

The real reason every thumbnail card stayed on the placeholder was a
reference-aliasing bug in the flush closure. `const batch = pendingUrls`
captured the same object; `delete (pendingUrls)[k]` for each key then
emptied `batch` too, because they were the same reference. By the time
`setThumbnailUrls(prev => ({ ...prev, ...batch }))` ran, batch was {}
and the state never picked up any URL — every card rendered the empty
placeholder icon.

Fixed by aliasing first, then replacing pendingUrls with a fresh empty
object (let instead of const on the outer binding). The cloned `batch`
retains its entries for the spread; any new arrivals during the commit
land in the new empty pendingUrls and coalesce into the next frame.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/desktop/src/components/ScreenSourcePicker.tsx

@@ -79,16 +79,18 @@ export function ScreenSourcePicker({ open, onClose, onStart }: Props) {
     // one setState — cuts re-renders from O(N) to O(frames) during the
     // initial fan-in and prevents consecutive 10-30 ms long tasks from
     // stacking in one frame.
-    const pendingUrls: Record<string, string> = {};
+    //
+    // Previously `batch` was aliased to `pendingUrls` and then we cleared
+    // pendingUrls via `delete` — which emptied batch too (same reference)
+    // and every flush ended up spreading nothing into the state. Clone
+    // first, then clear, so the batch keeps its entries.
+    let pendingUrls: Record<string, string> = {};
     let rafScheduled = false;
     const flush = () => {
       rafScheduled = false;
-      if (Object.keys(pendingUrls).length === 0) return;
       const batch = pendingUrls;
-      // Capture then reset so new arrivals during the commit land in a
-      // fresh batch instead of double-applying.
-      const keys = Object.keys(batch);
-      for (const k of keys) delete (pendingUrls as Record<string, string>)[k];
+      if (Object.keys(batch).length === 0) return;
+      pendingUrls = {};
       startTransition(() => {
         setThumbnailUrls((prev) => ({ ...prev, ...batch }));
       });