feat(studio): timeline revamp with active-clip highlighting and hide controls (#2017)

Timeline UI
- Highlight clips visible at the playhead in the primary color; others share one neutral color
- Minimalist rounded clips, single-color track rows, no gutter icons or superscript labels
- Per-track eye toggle and a per-element hide button in the design panel
- Ruler zoom fixes: sub-second tick intervals and correct label formatting at high zoom
- Sticky gutter so track controls stay visible while scrolling

WYSIWYG visibility (data-hidden)
- Runtime honors data-hidden (display:none), so hiding affects the render, not just the preview
- HTML stays the source of truth; hide state persists and round-trips on reload

Split several studio files to stay under the 600-line cap; pure relocations, no behavior change.

packages/studio/src/contexts/TimelineEditContext.tsx

@@ -32,6 +32,7 @@ export function TimelineEditProvider({
     [
       value.onMoveElement,
       value.onResizeElement,
+      value.onToggleTrackHidden,
       value.onBlockedEditAttempt,
       value.onSplitElement,
       value.onRazorSplit,