feat: sticky time ruler + tempo lane when scrolling tracks

- Wrap TimelineRuler + TempoTrackLane in sticky-top div
- Same sticky pattern for both main session and sub-tab views
- TCP column already has sticky header (top-0) and tempo row (top-10)
- Background color hides tracks scrolling behind ruler

app/static/js/app.jsx

@@ -15785,6 +15785,8 @@ const App = () => {
         width: `${timelineWidth}px`
       },
       className: "relative flex flex-col min-h-full"
+    }, /*#__PURE__*/React.createElement("div", {
+      className: "sticky top-0 z-30 bg-[#1a1a1a]"
     }, /*#__PURE__*/React.createElement(TimelineRuler, {
       bpm: parseInt(bpm) || 120,
       zoom: zoom,
@@ -15805,7 +15807,7 @@ const App = () => {
       onRulerMouseDown: handleRulerMouseDown,
       scrollLeft: scrollLeft,
       canvasRedrawCount: canvasRedrawCount
-    }), selectionMode === 'global' && selLeft !== null && selRight !== null && selRight > selLeft && /*#__PURE__*/React.createElement("div", {
+    })), selectionMode === 'global' && selLeft !== null && selRight !== null && selRight > selLeft && /*#__PURE__*/React.createElement("div", {
       className: "absolute inset-0 pointer-events-none z-20",
       style: {
         left: `${selLeft * zoom}px`,
@@ -16355,6 +16357,8 @@ const App = () => {
           width: `${subTabTimelineWidth}px`
         },
         className: "relative flex flex-col min-h-full"
+      }, /*#__PURE__*/React.createElement("div", {
+        className: "sticky top-0 z-30 bg-[#1a1a1a]"
       }, /*#__PURE__*/React.createElement(TimelineRuler, {
         bpm: parseInt(bpm) || 120,
         zoom: zoom,
@@ -16374,7 +16378,7 @@ const App = () => {
         },
         scrollLeft: scrollLeft,
         canvasRedrawCount: canvasRedrawCount
-      }), /*#__PURE__*/React.createElement("div", {
+      })), /*#__PURE__*/React.createElement("div", {
         className: "flex-1 flex flex-col relative bg-[#111111] min-h-full"
       }, vTrack && /*#__PURE__*/React.createElement("div", {
         style: {