refactor: Ctrl+click toggles selection, Ctrl+click+drag copies group

Changed behavior:
- Ctrl+Click on item (no drag): toggle selection (add/remove)
- Ctrl+Click+Drag on item (movement > 5px): copy selected group
- Alt+Click: move item immediately
- No modifier + click selected item: move group

Added pendingDragRef + useEffect to detect mousemove threshold
before starting copy-drag. onAddToSelection + onSetPendingDrag
props wired through WaveformLane.

app/static/js/app.jsx

@@ -388,6 +388,8 @@ const WaveformLane = ({
   getLocalAnchor,
   onClearLocalSelection,
   onDeselectItem,
+  onAddToSelection,
+  onSetPendingDrag,
   onSetSelectionMode,
   onSetSelectionStart,
   onSetSelectionEnd,
@@ -1080,10 +1082,18 @@ const WaveformLane = ({
       if (hitItem && !e.altKey && !e.shiftKey) {
         e.preventDefault();
         e.stopPropagation();
-        if (e.ctrlKey && selectedItemIds && selectedItemIds.has(hitItem.id)) {
+        if (e.ctrlKey) {
+          // Ctrl+Click: toggle selection only, never start drag immediately
+          if (selectedItemIds && selectedItemIds.has(hitItem.id)) {
             if (onDeselectItem) onDeselectItem(hitItem.id);
+          } else if (onAddToSelection) {
+            onAddToSelection(hitItem.id);
+          }
+          // Store pending drag so mousemove can start copy-drag
+          if (onSetPendingDrag) onSetPendingDrag(track.id, hitItem.type, hitItem.id, time - hitItem.start, e.nativeEvent || e);
         } else {
-          if (onSectionItemDragStart) onSectionItemDragStart(track.id, hitItem.type, hitItem.id, time - hitItem.start, e.ctrlKey);
+          // No modifier: start drag immediately (single or multi move)
+          if (onSectionItemDragStart) onSectionItemDragStart(track.id, hitItem.type, hitItem.id, time - hitItem.start, false);
         }
         return;
       }
@@ -1139,15 +1149,18 @@ const WaveformLane = ({
       if (clickedClip && (e.altKey || e.ctrlKey)) {
         e.preventDefault();
         e.stopPropagation();
-        if (e.ctrlKey && selectedItemIds) {
+        if (e.ctrlKey) {
+          // Ctrl+Click: toggle selection, store pending drag
           var clipCanonicalId = clickedClip.id === 'default' ? 'default_' + track.id : clickedClip.id;
-          if (selectedItemIds.has(clipCanonicalId)) {
+          if (selectedItemIds && selectedItemIds.has(clipCanonicalId)) {
             if (onDeselectItem) onDeselectItem(clipCanonicalId);
-            return;
+          } else if (onAddToSelection) {
+            onAddToSelection(clipCanonicalId);
           }
-        }
+          if (onSetPendingDrag) onSetPendingDrag(track.id, 'clip', clipCanonicalId, time - clickedClip.startTime, e.nativeEvent || e);
-        if (onClipDragStart) {
+        } else {
-          onClipDragStart(track.id, clickedClip.id, time - clickedClip.startTime, e.ctrlKey);
+          // Alt+Click: move immediately
+          if (onClipDragStart) onClipDragStart(track.id, clickedClip.id, time - clickedClip.startTime, false);
         }
         return;
       }
@@ -6934,6 +6947,7 @@ const App = () => {
   const sweepStartRef = useRef(0);
   const sweepTrackIdRef = useRef(null);
   const sweepSelectRef = useRef(null);
+  const pendingDragRef = useRef(null); // { trackId, itemType, itemId, clickOffset, startX, startY }
   const [localSelectionTrackId, setLocalSelectionTrackId] = useState(null);
   const [localSelectionStart, setLocalSelectionStart] = useState(null);
   const [localSelectionEnd, setLocalSelectionEnd] = useState(null);
@@ -11641,6 +11655,18 @@ const App = () => {
     });
   };
 
+  // ── Add item to selectedItemIds ──
+  const handleAddToSelection = itemId => {
+    setSelectedItemIds(prev => {
+      var next = new Set(prev);
+      next.add(itemId);
+      return next;
+    });
+  };
+  const handleSetPendingDrag = (trackId, itemType, itemId, clickOffset, e) => {
+    pendingDragRef.current = { trackId, itemType, itemId, clickOffset, startX: e.clientX, startY: e.clientY };
+  };
+
   // ── Sweep Select ──
   const handleSweepSelectStart = (trackId, startTime) => {
     isSweepingRef.current = true;
@@ -11841,6 +11867,27 @@ const App = () => {
     };
   }, [zoom, activeTab, sessionTabs]);
 
+  // ── Pending drag: Ctrl+click toggles selection; mousemove > threshold starts copy-drag ──
+  useEffect(() => {
+    const handleMouseMove = e => {
+      var pd = pendingDragRef.current;
+      if (!pd) return;
+      var dx = e.clientX - pd.startX;
+      if (Math.abs(dx) > 5) {
+        pendingDragRef.current = null;
+        handleSectionItemDragStart(pd.trackId, pd.itemType, pd.itemId, pd.clickOffset, true);
+      }
+    };
+    document.addEventListener('mousemove', handleMouseMove);
+    var handleMouseUp = function() { pendingDragRef.current = null; };
+    document.addEventListener('mouseup', handleMouseUp);
+    return function() {
+      document.removeEventListener('mousemove', handleMouseMove);
+      document.removeEventListener('mouseup', handleMouseUp);
+      pendingDragRef.current = null;
+    };
+  }, []);
+
   // ── Document-level mousemove/mouseup for Section/MIDI item resize ──
   useEffect(() => {
     const handleMouseMove = e => {
@@ -16522,6 +16569,8 @@ const App = () => {
          onTrackLaneMouseDown: handleTrackLaneMouseDown,
          onSweepSelectStart: handleSweepSelectStart,
          onDeselectItem: handleDeselectItem,
+         onAddToSelection: handleAddToSelection,
+         onSetPendingDrag: handleSetPendingDrag,
         onContextMenu: handleContextMenu,
         onClipDragStart: handleClipDragStart,
         onClipStretchStart: handleClipStretchStart,