feat: add intuitive drag-to-select on waveforms

Improved selection UX to match professional DAWs:
- Drag directly on waveform to create selections (no modifier keys needed)
- Click without dragging seeks playhead and clears selection
- 3-pixel drag threshold prevents accidental selections
- Fixed variable name conflict with existing file drag-and-drop feature

Users can now naturally drag across waveforms to select regions for
editing, providing a more intuitive workflow.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

components/tracks/Track.tsx

@@ -68,6 +68,8 @@ export function Track({
   // Selection state
   const [isSelecting, setIsSelecting] = React.useState(false);
   const [selectionStart, setSelectionStart] = React.useState<number | null>(null);
+  const [isSelectingByDrag, setIsSelectingByDrag] = React.useState(false);
+  const [dragStartPos, setDragStartPos] = React.useState<{ x: number; y: number } | null>(null);
 
   const handleNameClick = () => {
     setIsEditingName(true);
@@ -224,41 +226,66 @@ export function Track({
 
     const rect = e.currentTarget.getBoundingClientRect();
     const x = e.clientX - rect.left;
+    const y = e.clientY - rect.top;
     const clickTime = (x / rect.width) * duration;
 
-    // Shift+drag to create selection
-    if (e.shiftKey) {
-      setIsSelecting(true);
-      setSelectionStart(clickTime);
-      onSelectionChange?.({ start: clickTime, end: clickTime });
-    } else {
-      // Regular click clears selection and seeks
-      onSelectionChange?.(null);
-      if (onSeek) {
-        onSeek(clickTime);
-      }
-    }
+    // Store drag start position
+    setDragStartPos({ x: e.clientX, y: e.clientY });
+    setIsSelectingByDrag(false);
+
+    // Start selection immediately (will be used if user drags)
+    setIsSelecting(true);
+    setSelectionStart(clickTime);
   };
 
   const handleCanvasMouseMove = (e: React.MouseEvent<HTMLCanvasElement>) => {
-    if (!isSelecting || selectionStart === null || !duration) return;
+    if (!isSelecting || selectionStart === null || !duration || !dragStartPos) return;
 
     const rect = e.currentTarget.getBoundingClientRect();
     const x = e.clientX - rect.left;
     const currentTime = (x / rect.width) * duration;
 
-    // Clamp to valid time range
-    const clampedTime = Math.max(0, Math.min(duration, currentTime));
+    // Check if user has moved enough to be considered dragging (threshold: 3 pixels)
+    const dragDistance = Math.sqrt(
+      Math.pow(e.clientX - dragStartPos.x, 2) + Math.pow(e.clientY - dragStartPos.y, 2)
+    );
 
-    // Update selection (ensure start < end)
-    const start = Math.min(selectionStart, clampedTime);
-    const end = Math.max(selectionStart, clampedTime);
+    if (dragDistance > 3) {
+      setIsSelectingByDrag(true);
+    }
 
-    onSelectionChange?.({ start, end });
+    // If dragging, update selection
+    if (isSelectingByDrag || dragDistance > 3) {
+      // Clamp to valid time range
+      const clampedTime = Math.max(0, Math.min(duration, currentTime));
+
+      // Update selection (ensure start < end)
+      const start = Math.min(selectionStart, clampedTime);
+      const end = Math.max(selectionStart, clampedTime);
+
+      onSelectionChange?.({ start, end });
+    }
   };
 
-  const handleCanvasMouseUp = () => {
+  const handleCanvasMouseUp = (e: React.MouseEvent<HTMLCanvasElement>) => {
+    if (!duration) return;
+
+    const rect = e.currentTarget.getBoundingClientRect();
+    const x = e.clientX - rect.left;
+    const clickTime = (x / rect.width) * duration;
+
+    // If user didn't drag (just clicked), clear selection and seek
+    if (!isSelectingByDrag) {
+      onSelectionChange?.(null);
+      if (onSeek) {
+        onSeek(clickTime);
+      }
+    }
+
+    // Reset drag state
     setIsSelecting(false);
+    setIsSelectingByDrag(false);
+    setDragStartPos(null);
   };
 
   // Handle mouse leaving canvas during selection
@@ -266,6 +293,8 @@ export function Track({
     const handleGlobalMouseUp = () => {
       if (isSelecting) {
         setIsSelecting(false);
+        setIsSelectingByDrag(false);
+        setDragStartPos(null);
       }
     };