fix: shift+scroll velocity changes all selected notes

When selectedNoteIds.length > 0, shift+scroll on any note changes
velocity for all selected notes. Also fixed hit-test to subtract
renderBeatOffset for correct positioning in session mode.

app/static/js/app.jsx

@@ -4810,11 +4810,16 @@ const PianoRollTabEditor = ({ st, zoom, bpm, viewportWidth, activeTracks, onClos
 
       if (e.shiftKey) {
         e.preventDefault();
-        // Shift+scroll on note → change velocity
-        const clickedNote = notes.find(n => pitch === n.pitch && mx >= n.start_beat * pixelsPerBeat && mx < (n.start_beat + n.duration_beats) * pixelsPerBeat);
+        // Shift+scroll on note → change velocity of single note or all selected
+        const scrollBeat = (mx / pixelsPerBeat) - renderBeatOffset;
+        const clickedNote = notes.find(n => pitch === n.pitch && scrollBeat >= n.start_beat && scrollBeat < (n.start_beat + n.duration_beats));
         if (clickedNote) {
           const delta = e.deltaY < 0 ? 0.05 : -0.05;
-          setNotes(prev => prev.map(n => n.id === clickedNote.id ? { ...n, velocity: Math.max(0.05, Math.min(1, (n.velocity || 0.8) + delta)) } : n));
+          if (selectedNoteIds.length > 0) {
+            setNotes(prev => prev.map(n => selectedNoteIds.includes(n.id) ? { ...n, velocity: Math.max(0.05, Math.min(1, (n.velocity || 0.8) + delta)) } : n));
+          } else {
+            setNotes(prev => prev.map(n => n.id === clickedNote.id ? { ...n, velocity: Math.max(0.05, Math.min(1, (n.velocity || 0.8) + delta)) } : n));
+          }
         } else {
           // Shift+scroll on empty space → horizontal scroll
           const container = gridScrollRef.current;
@@ -8294,7 +8299,7 @@ const App = () => {
       buffer: silentBuffer,
       instrumentProgram: track.instrumentProgram,
       instrumentName: track.instrumentName,
-      currentTime: Math.max(0, currentTime - midiItem.startTime),
+      currentTime: 0,
       isPlaying: false,
       selectionStart: null,
       selectionEnd: null,