feat: add click-to-load audio on empty track waveform

Changes:
- Empty tracks now show upload icon and "Click to load audio file" message
- Clicking the placeholder opens native file dialog
- Automatically decodes audio file and updates track with AudioBuffer
- Auto-renames track to filename if track name is still default
- Hover effect with background color change for better UX
- Message about drag & drop coming soon

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

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

components/tracks/Track.tsx

@@ -1,7 +1,7 @@
 'use client';
 
 import * as React from 'react';
-import { Volume2, VolumeX, Headphones, Trash2, ChevronDown, ChevronRight, CircleArrowOutUpRight } from 'lucide-react';
+import { Volume2, VolumeX, Headphones, Trash2, ChevronDown, ChevronRight, CircleArrowOutUpRight, Upload } from 'lucide-react';
 import type { Track as TrackType } from '@/types/track';
 import { Button } from '@/components/ui/Button';
 import { Slider } from '@/components/ui/Slider';
@@ -22,6 +22,7 @@ export interface TrackProps {
   onRemove: () => void;
   onNameChange: (name: string) => void;
   onSeek?: (time: number) => void;
+  onLoadAudio?: (buffer: AudioBuffer) => void;
 }
 
 export function Track({
@@ -39,9 +40,11 @@ export function Track({
   onRemove,
   onNameChange,
   onSeek,
+  onLoadAudio,
 }: TrackProps) {
   const canvasRef = React.useRef<HTMLCanvasElement>(null);
   const containerRef = React.useRef<HTMLDivElement>(null);
+  const fileInputRef = React.useRef<HTMLInputElement>(null);
   const [isEditingName, setIsEditingName] = React.useState(false);
   const [nameInput, setNameInput] = React.useState(String(track.name || 'Untitled Track'));
   const inputRef = React.useRef<HTMLInputElement>(null);
@@ -162,6 +165,33 @@ export function Track({
     onSeek(clickTime);
   };
 
+  const handleFileChange = async (e: React.ChangeEvent<HTMLInputElement>) => {
+    const file = e.target.files?.[0];
+    if (!file || !onLoadAudio) 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);
+    }
+
+    // Reset input
+    e.target.value = '';
+  };
+
+  const handleLoadAudioClick = () => {
+    fileInputRef.current?.click();
+  };
+
   const trackHeight = track.collapsed ? 48 : track.height;
 
   return (
@@ -322,9 +352,26 @@ export function Track({
                 onClick={handleCanvasClick}
               />
             ) : (
-              <div className="absolute inset-0 flex items-center justify-center text-sm text-muted-foreground">
-                No audio loaded
-              </div>
+              <>
+                <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"
+                  onClick={(e) => {
+                    e.stopPropagation();
+                    handleLoadAudioClick();
+                  }}
+                >
+                  <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>
+                </div>
+                <input
+                  ref={fileInputRef}
+                  type="file"
+                  accept="audio/*"
+                  onChange={handleFileChange}
+                  className="hidden"
+                />
+              </>
             )}
           </>
         )}

components/tracks/TrackList.tsx

@@ -104,6 +104,9 @@ export function TrackList({
               onUpdateTrack(track.id, { name })
             }
             onSeek={onSeek}
+            onLoadAudio={(buffer) =>
+              onUpdateTrack(track.id, { audioBuffer: buffer })
+            }
           />
         ))}
       </div>