feat: add MIDI soundfont preferences

Extend MidiPreferences to support configurable soundfonts for both
MIDI.js and SpessaSynth synthesizers.

- Add midiJsSoundfont field with MusyngKite as default
- Add spessaSynthSoundfont field for custom SF2/SF3 URLs
- Support user-configurable synthesis quality vs performance
- Enables single MIDI.js port with multiple soundfont options

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

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

src/PreferencesStore.tsx

@@ -35,6 +35,8 @@ export type MidiPreferences = {
   enabled: boolean;
   lastInputDeviceId?: string;
   lastOutputDeviceId?: string;
+  midiJsSoundfont: string;
+  spessaSynthSoundfont: string;
 };
 
 export type PrefState = {
@@ -114,6 +116,8 @@ class PreferencesStore {
       },
       midi: {
         enabled: false,
+        midiJsSoundfont: 'MusyngKite', // Default to MusyngKite
+        spessaSynthSoundfont: '', // Empty default, user will need to provide soundfont URL
       },
     };
   }