Skip leading silence in preview playback

Normalize event times to start at 0 in SoundfontMIDIPlayer,
matching the existing fix in SynthesisEngine. Prevents delay
when MIDI files have silence before the first note.

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/synthesis/SoundfontMIDIPlayer.ts

@@ -25,7 +25,11 @@ export class SoundfontMIDIPlayer {
   }
 
   async load(events: NoteEvent[]): Promise<void> {
-    this.events = [...events].sort((a, b) => a.time - b.time);
+    const sorted = [...events].sort((a, b) => a.time - b.time);
+
+    // Normalize times to start at 0 (skip leading silence)
+    const minTime = sorted.length > 0 ? sorted[0].time : 0;
+    this.events = sorted.map(e => ({ ...e, time: e.time - minTime }));
     this.currentEventIndex = 0;
     
     // Load the acoustic grand piano instrument (most versatile for MIDI playback)