Auto-stop Motif playback on search/song change

- Stop Motif audio when starting a new search
- Stop Motif audio when selecting a different song
- Prevents overlapping audio from previous generations

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

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

src/main.ts

@@ -142,6 +142,9 @@ class MotifApp {
     const songName = this.songInput.value.trim();
     if (!songName) return;
 
+    // Stop any playing Motif audio
+    this.handleMotifStop();
+
     this.updateStatus('Searching for MIDI files...');
     this.searchBtn.disabled = true;
     this.hideResults();
@@ -208,6 +211,9 @@ class MotifApp {
   public async selectResult(index: number): Promise<void> {
     if (index < 0 || index >= this.searchResults.length) return;
 
+    // Stop any playing Motif audio
+    this.handleMotifStop();
+
     this.selectedResultIndex = index;
     const result = this.searchResults[index];