feat: brush-pen scale snap + scale sub-menu hover + keybed swipe

- Brush (pen tool only) draws notes snapped to selected scale
- Scale sub-menus open on hover instead of click
- Keybed: swipe/drag across keys plays notes (mouseenter+mousedown)
- Global mouseup resets keybed drag state

app/static/js/app.jsx

@@ -4554,7 +4554,13 @@ const PianoRollTabEditor = ({ st, zoom, bpm, viewportWidth, onClose, onUpdateNot
   const ccWrapperRef = React.useRef(null);
   const gridScrollRef = React.useRef(null);
   const keybedRef = React.useRef(null);
+  const keybedMouseDownRef = React.useRef(false);
   const rulerScrollRef = React.useRef(null);
+  React.useEffect(() => {
+    const up = () => { keybedMouseDownRef.current = false; };
+    window.addEventListener('mouseup', up);
+    return () => window.removeEventListener('mouseup', up);
+  }, []);
 
   const NoteHeight = 18;
   const PITCH_START = 0; // C0 (render all 128 keys)
@@ -5043,8 +5049,8 @@ const PianoRollTabEditor = ({ st, zoom, bpm, viewportWidth, onClose, onUpdateNot
         startOffsetPitch: pitch - clickedNote.pitch,
         selectedNotesOffset: selectedNotesOffset
       });
-    } else {
-      // Click on empty space: DRAW a new note (brush mode with visitedPitches)
+    } else if (activeRollTool === 'pen') {
+      // Click on empty space with pen tool: DRAW a new note (brush mode with visitedPitches)
       pushToUndo(notes);
       const start = getSnapBeat(beat, snapVal);
       const initialDur = getSnapDuration(snapVal);
@@ -5340,6 +5346,7 @@ const PianoRollTabEditor = ({ st, zoom, bpm, viewportWidth, onClose, onUpdateNot
           className: `w-[60px] shrink-0 border-b border-zinc-800 text-[9px] font-bold flex items-center justify-end pr-2 transition cursor-pointer ${isBlack ? 'bg-zinc-950 text-slate-500 border-zinc-900 hover:bg-zinc-800' : 'bg-white text-zinc-800 border-r border-zinc-400 hover:bg-amber-100'}`,
           onMouseDown: (e) => {
             e.stopPropagation();
+            keybedMouseDownRef.current = true;
             try {
               if (window.SonicSF) {
                 window.SonicSF.playNote(pitch, 100, 500, undefined, st.instrumentProgram, null);
@@ -5348,6 +5355,16 @@ const PianoRollTabEditor = ({ st, zoom, bpm, viewportWidth, onClose, onUpdateNot
               console.error('playNote error:', err);
             }
           },
+          onMouseEnter: (e) => {
+            if (keybedMouseDownRef.current) {
+              try {
+                if (window.SonicSF) {
+                  window.SonicSF.playNote(pitch, 100, 200, undefined, st.instrumentProgram, null);
+                }
+              } catch (err) { console.error('playNote error:', err); }
+            }
+          },
+          onMouseUp: () => { keybedMouseDownRef.current = false; },
           onContextMenu: (e) => {
             e.preventDefault();
             e.stopPropagation();
@@ -5498,11 +5515,12 @@ const PianoRollTabEditor = ({ st, zoom, bpm, viewportWidth, onClose, onUpdateNot
     const closeMenu = () => setScaleMenuPos(null);
     const items = [];
     const isSameScale = (a, b) => { if (!a || !b) return a === b; if (a.length !== b.length) return false; return a.every((v,i)=>v===b[i]); };
-    const pushItem = (label, onClick, indent) => {
+    const pushItem = (label, onClick, indent, onHover) => {
       const isActive = onClick._scale && isSameScale(onClick._scale, selectedScale);
       items.push(React.createElement("div", {
         key: label,
         onClick: () => { onClick(); closeMenu(); },
+        onMouseEnter: onHover || undefined,
         className: "px-3 py-1.5 text-xs cursor-pointer hover:bg-amber-600 hover:text-white whitespace-nowrap " + (indent ? "pl-6 " : "") + (isActive ? "bg-amber-800/40 text-amber-300" : "text-zinc-300")
       }, label));
     };
@@ -5514,7 +5532,9 @@ const PianoRollTabEditor = ({ st, zoom, bpm, viewportWidth, onClose, onUpdateNot
       } else {
         const parentKey = key;
         const isOpen = scaleMenuPos && scaleMenuPos.parentKey === parentKey;
-        pushItem(key + " ▸", () => { setScaleMenuPos({ x: scaleMenuPos.x + 120, y: scaleMenuPos.y, parentKey }); }, false);
+        pushItem(key + " ▸", () => setSelectedScale(null), false, () => {
+          setScaleMenuPos({ x: scaleMenuPos.x + 120, y: scaleMenuPos.y, parentKey });
+        });
         if (isOpen) {
           Object.keys(val).forEach(subKey => {
             pushItem(subKey, () => setSelectedScale(val[subKey]), true);