fix: restore waveform click-to-seek functionality

Re-enabled mouse event handlers on waveform canvas that were accidentally
removed. Users can now:
- Click to seek to a specific position
- Drag to create selection regions

Also fixed TypeScript error by properly typing EffectType parameter in
handleAddEffect callback.

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

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

components/tracks/Track.tsx

@@ -628,7 +628,10 @@ export function Track({
             {/* Waveform Canvas */}
             <canvas
               ref={canvasRef}
-              className="absolute inset-0 w-full h-full"
+              className="absolute inset-0 w-full h-full cursor-pointer"
+              onMouseDown={handleCanvasMouseDown}
+              onMouseMove={handleCanvasMouseMove}
+              onMouseUp={handleCanvasMouseUp}
             />
           </>
         ) : (