feat: SF3 conversion + SpessaSynth client player

Server:
- soundfont_converter.py: Python SF2->SF3 via ffmpeg Ogg compression
- batch_convert_all runs on startup (daemon thread)
- GET /soundfonts/download/{sf_id} serves SF3 with SF2 fallback
- Dockerfile: add fluidsynth, vorbis-tools

Client:
- soundfontStorage.js: IndexedDB cache for SF3 buffers
- soundfontPlayer.js: dual-mode (SpessaSynth + oscillator fallback)
- app.jsx: init SpessaSynth, loadSF on instrument select
- index.html: SpessaSynth CDN import + storage script tag

Compression: DSK 11M->1.1M (90%), SGM 529M->18M (97%)

app/static/js/app.jsx

@@ -37,6 +37,9 @@ function getAudioContext() {
     if (window.SonicAudio && window.SonicAudio.initAudioWorklet) {
       window.SonicAudio.initAudioWorklet();
     }
+    if (window.SonicSF && window.SonicSF.init) {
+      window.SonicSF.init(audioCtx);
+    }
   }
   if (audioCtx.state === 'suspended') {
     audioCtx.resume();
@@ -6519,6 +6522,11 @@ const App = () => {
     setInstrumentSelectorTrackId(null);
     setSynthCategory(null);
     setSelectedSoundFontId(null);
+    // Trigger SpessaSynth SF3 load when soundfont instrument selected
+    if (window.SonicSF && window.SonicSF.loadSoundFont && instrumentId && typeof instrumentId === 'string' && instrumentId.startsWith('sf_')) {
+      const sfId = instrumentId.replace('sf_', '');
+      window.SonicSF.loadSoundFont(sfId);
+    }
     setSubTabs(prev => prev.map(s => {
       if (s.trackId !== trackId) return s;
       return { ...s, instrumentProgram: programNumber !== undefined ? programNumber : undefined, instrumentName: displayName, instrumentId };
@@ -13867,6 +13875,9 @@ const App = () => {
               if (window.SonicSF && window.SonicSF.applyAITrackInstrument) {
                 window.SonicSF.applyAITrackInstrument(aiTrack.soundfont_bank, aiTrack.soundfont_program);
               }
+              if (window.SonicSF && window.SonicSF.loadSoundFont && aiTrack.soundfont_id) {
+                window.SonicSF.loadSoundFont(aiTrack.soundfont_id);
+              }
             }
           });
           return updatedTracks;