feat(studio): drag a clip past the video end to extend its duration

Dragging a timeline clip (or its right resize edge) past the end of the
video now extends the composition duration on drop, instead of clamping
the clip at the current end. Extend-only and undoable.

- Relax the move/resize horizontal clamps that pinned a clip's end at the
  current duration; effectiveDuration now folds in the active drag/resize
  preview so the ruler and track width grow live as you drag past the end.
- On drop, extend the root composition data-duration (and the store) when
  the clip's new end exceeds it, via a shared extendRootDurationInSource
  helper extracted from the block installer (now the single owner of that
  logic). An extending edit routes through the server persist path since
  the SDK setTiming op can't express the root composition's own duration.

packages/studio/src/player/components/Timeline.tsx

@@ -180,14 +180,6 @@ export const Timeline = memo(function Timeline({
     if (shortcutHintRafRef.current) cancelAnimationFrame(shortcutHintRafRef.current);
   });
 
-  const effectiveDuration = useMemo(() => {
-    const safeDur = Number.isFinite(duration) ? duration : 0;
-    if (rawElements.length === 0) return safeDur;
-    const maxEnd = Math.max(...rawElements.map((el) => el.start + el.duration));
-    const result = Math.max(safeDur, maxEnd);
-    return Number.isFinite(result) ? result : safeDur;
-  }, [rawElements, duration]);
-
   const tracks = useMemo(
     () => buildStackingTimelineLayers(expandedElements).rows,
     [expandedElements],
@@ -210,8 +202,7 @@ export const Timeline = memo(function Timeline({
   expandedElementsRef.current = expandedElements;
 
   const ppsRef = useRef(100);
-  const durationRef = useRef(effectiveDuration);
-  durationRef.current = effectiveDuration;
+  const durationRef = useRef(Number.isFinite(duration) ? duration : 0);
 
   // Stable ref so useTimelineClipDrag can clear rangeSelection without circular dep
   const setRangeSelectionRef = useRef<((sel: null) => void) | null>(null);
@@ -227,7 +218,6 @@ export const Timeline = memo(function Timeline({
   } = useTimelineClipDrag({
     scrollRef,
     ppsRef,
-    durationRef,
     trackOrderRef,
     timelineLayersRef,
     timelineElementsRef: expandedElementsRef,
@@ -238,6 +228,22 @@ export const Timeline = memo(function Timeline({
     setRangeSelectionRef,
   });
 
+  const effectiveDuration = useMemo(() => {
+    const safeDur = Number.isFinite(duration) ? duration : 0;
+    let maxEnd = safeDur;
+    if (rawElements.length > 0) {
+      maxEnd = Math.max(maxEnd, ...rawElements.map((el) => el.start + el.duration));
+    }
+    if (draggedClip?.started) {
+      maxEnd = Math.max(maxEnd, draggedClip.previewStart + draggedClip.element.duration);
+    }
+    if (resizingClip?.started) {
+      maxEnd = Math.max(maxEnd, resizingClip.previewStart + resizingClip.previewDuration);
+    }
+    return Number.isFinite(maxEnd) ? maxEnd : safeDur;
+  }, [rawElements, duration, draggedClip, resizingClip]);
+  durationRef.current = effectiveDuration;
+
   const displayTrackOrder = useMemo(() => {
     if (
       !draggedClip?.started ||