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/App.tsx

@@ -360,7 +360,6 @@ export function StudioApp() {
     resetErrors: resetConsoleErrors,
   } = useConsoleErrorCapture(previewIframe);
   const dragOverlay = useDragOverlay(fileManager.handleImportFiles);
-  // Gesture recording
   const handleToggleRecordingRef = useRef<() => void>(() => {});
   const domEditSessionRef = useRef(domEditSession);
   domEditSessionRef.current = domEditSession;
@@ -531,6 +530,7 @@ export function StudioApp() {
                       handleTimelineFileDrop={timelineEditing.handleTimelineFileDrop}
                       handleTimelineElementMove={timelineEditing.handleTimelineElementMove}
                       handleTimelineElementResize={timelineEditing.handleTimelineElementResize}
+                      handleToggleTrackHidden={timelineEditing.handleToggleTrackHidden}
                       handleBlockedTimelineEdit={timelineEditing.handleBlockedTimelineEdit}
                       handleTimelineElementSplit={timelineEditing.handleTimelineElementSplit}
                       handleRazorSplit={timelineEditing.handleRazorSplit}
@@ -572,6 +572,7 @@ export function StudioApp() {
                         domEditSaveTimestampRef={domEditSaveTimestampRef}
                         recordEdit={editHistory.recordEdit}
                         {...cropModeProps}
+                        onToggleElementHidden={timelineEditing.handleToggleElementHidden}
                       />
                     )}
                   </div>