fix: subtract renderBeatOffset from mouse beat for correct positioning

- handleGridMouseDown/move: beat = x/pixelsPerBeat - renderBeatOffset
  so hit-testing and note creation use item-relative coordinates
- CC mouse handlers: same offset subtraction
- Ruler click: clickTime subtracts renderBeatOffset so st.currentTime
  remains item-relative

app/static/js/app.jsx

@@ -5076,7 +5076,7 @@ const PianoRollTabEditor = ({ st, zoom, bpm, viewportWidth, activeTracks, onClos
     const x = e.clientX - rect.left;
     const y = e.clientY - rect.top;
 
-    const beat = x / pixelsPerBeat;
+    const beat = x / pixelsPerBeat - renderBeatOffset;
     const pitch = 127 - Math.floor(y / NoteHeight);
 
     if (scaleMenuPos) setScaleMenuPos(null);
@@ -5261,7 +5261,7 @@ const PianoRollTabEditor = ({ st, zoom, bpm, viewportWidth, activeTracks, onClos
     const x = e.clientX - rect.left;
     const y = e.clientY - rect.top;
 
-    const beat = x / pixelsPerBeat;
+    const beat = x / pixelsPerBeat - renderBeatOffset;
     const pitch = 127 - Math.floor(y / NoteHeight);
 
     if (selectionMarquee) {
@@ -5536,7 +5536,7 @@ const PianoRollTabEditor = ({ st, zoom, bpm, viewportWidth, activeTracks, onClos
     const x = e.clientX - rect.left;
     const y = e.clientY - rect.top;
     const h = rect.height;
-    const beat = x / pixelsPerBeat;
+    const beat = x / pixelsPerBeat - renderBeatOffset;
 
     const noteIdx = findCCNoteIndex(beat, y, h);
     const val = Math.max(0, Math.min(1, (h - y) / h));
@@ -5588,7 +5588,7 @@ const PianoRollTabEditor = ({ st, zoom, bpm, viewportWidth, activeTracks, onClos
     const x = e.clientX - rect.left;
     const y = e.clientY - rect.top;
     const h = rect.height;
-    const beat = x / pixelsPerBeat;
+    const beat = x / pixelsPerBeat - renderBeatOffset;
     const val = Math.max(0, Math.min(1, (h - y) / h));
 
     const drag = ccDragRef.current;
@@ -6009,7 +6009,7 @@ const beatSec = 60.0 / (parseInt(bpm) || 120);
         const rect = e.currentTarget.getBoundingClientRect();
         const x = e.clientX - rect.left + e.currentTarget.scrollLeft;
         const clickBeat = x / pixelsPerBeat;
-        const clickTime = clickBeat * beatSec;
+        const clickTime = (clickBeat - renderBeatOffset) * beatSec;
         const clickInRange = loopStartBeat !== null && loopEndBeat !== null && clickBeat >= loopStartBeat && clickBeat <= loopEndBeat;
         if (e.ctrlKey || e.metaKey) {
           setLoopStartBeat(null);