fix(studio): keep the timeline multi-selection through a group edit

setSelectedElementId no longer resets the selection set when re-selecting an element
that is already a member: DOM-to-selection sync echoes fire on every pointer move during
a group drag and were collapsing the set to the grabbed clip.

Also drops the duplicate clearSelectedElementIds action in favor of clearSelection,
which the marquee now uses to clear on an empty drag.

packages/studio/src/player/components/useTimelineMarqueeSelection.ts

@@ -231,7 +231,7 @@ export function useTimelineMarqueeSelection({
       setMarqueeRect(null);
 
       if (!active.started) {
-        usePlayerStore.getState().setSelection([]);
+        usePlayerStore.getState().clearSelection();
         return true;
       }