feat: implement drag & drop for audio loading and decrease upload icon

Changes:
- Reduced upload icon size from h-8 w-8 to h-6 w-6 (smaller, cleaner)
- Implemented full drag & drop functionality for audio files
- Shows visual feedback when dragging: blue border, primary color highlight
- Changes text to "Drop audio file here" when dragging over
- Validates dropped file is audio type before processing
- Updated message from "coming soon" to active "or drag & drop"

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

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

components/tracks/Track.tsx

@@ -192,6 +192,50 @@ export function Track({
     fileInputRef.current?.click();
   };
 
+  const [isDragging, setIsDragging] = React.useState(false);
+
+  const handleDragOver = (e: React.DragEvent) => {
+    e.preventDefault();
+    e.stopPropagation();
+    setIsDragging(true);
+  };
+
+  const handleDragLeave = (e: React.DragEvent) => {
+    e.preventDefault();
+    e.stopPropagation();
+    setIsDragging(false);
+  };
+
+  const handleDrop = async (e: React.DragEvent) => {
+    e.preventDefault();
+    e.stopPropagation();
+    setIsDragging(false);
+
+    const file = e.dataTransfer.files?.[0];
+    if (!file || !onLoadAudio) return;
+
+    // Check if it's an audio file
+    if (!file.type.startsWith('audio/')) {
+      console.warn('Dropped file is not an audio file');
+      return;
+    }
+
+    try {
+      const arrayBuffer = await file.arrayBuffer();
+      const audioContext = new AudioContext();
+      const audioBuffer = await audioContext.decodeAudioData(arrayBuffer);
+      onLoadAudio(audioBuffer);
+
+      // Update track name to filename if it's still default
+      if (track.name === 'New Track' || track.name === 'Untitled Track') {
+        const fileName = file.name.replace(/\.[^/.]+$/, '');
+        onNameChange(fileName);
+      }
+    } catch (error) {
+      console.error('Failed to load audio file:', error);
+    }
+  };
+
   const trackHeight = track.collapsed ? 48 : track.height;
 
   return (
@@ -354,15 +398,21 @@ export function Track({
             ) : (
               <>
                 <div
-                  className="absolute inset-0 flex flex-col items-center justify-center text-sm text-muted-foreground hover:text-foreground hover:bg-accent/50 transition-colors cursor-pointer group"
+                  className={cn(
+                    "absolute inset-0 flex flex-col items-center justify-center text-sm text-muted-foreground hover:text-foreground transition-colors cursor-pointer group",
+                    isDragging ? "bg-primary/20 text-primary border-2 border-primary border-dashed" : "hover:bg-accent/50"
+                  )}
                   onClick={(e) => {
                     e.stopPropagation();
                     handleLoadAudioClick();
                   }}
+                  onDragOver={handleDragOver}
+                  onDragLeave={handleDragLeave}
+                  onDrop={handleDrop}
                 >
-                  <Upload className="h-8 w-8 mb-2 opacity-50 group-hover:opacity-100" />
-                  <p>Click to load audio file</p>
-                  <p className="text-xs opacity-75 mt-1">or drag & drop (coming soon)</p>
+                  <Upload className="h-6 w-6 mb-2 opacity-50 group-hover:opacity-100" />
+                  <p>{isDragging ? 'Drop audio file here' : 'Click to load audio file'}</p>
+                  <p className="text-xs opacity-75 mt-1">or drag & drop</p>
                 </div>
                 <input
                   ref={fileInputRef}