feat(studio): razor/blade tool UI for timeline clip splitting (#1331)

Wire the razor tool into Studio's timeline UI:

- B enters razor mode (crosshair cursor + red vertical guide line)
- Click any clip to split at the click position
- Shift+click splits all clips across every track at that time
- V or Escape exits razor mode
- Toolbar shows selection arrow / scissors toggle

Add useRazorSplit hook for split orchestration (HTML + GSAP mutation).
Add activeTool state to playerStore. Add preview reload after timeline
move/resize operations so the composition re-renders with updated timing.

packages/studio/src/utils/timelineElementSplit.ts

@@ -2,6 +2,9 @@ import type { TimelineElement } from "../player/store/playerStore";
 
 export { buildPatchTarget, readFileContent } from "../hooks/timelineEditingHelpers";
 
+/** Minimum distance (seconds) from clip boundaries to allow a split. */
+export const SPLIT_BOUNDARY_EPSILON_S = 0.03;
+
 export function canSplitElement(el: TimelineElement): boolean {
   return (
     !el.timelineLocked &&