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/TrackList.tsx

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