fix: implement proper init chain + AudioContext.resume + _initPromise

Per md/38_FLOWCLIENT.md analysis:
- Add _initPromise to serialize concurrent init calls (no duplicate worklet)
- audioContext.resume() before worklet.loadModule
- selectInstrument creates AudioContext + awaits init if not ready
- playNote tries SpessaSynth first, falls back to oscillator on error
- All MIDI commands wrapped in try/catch with oscillator fallback

app/static/js/services/soundfontPlayer.js

@@ -2,8 +2,8 @@
     'use strict';
 
     const activeOscillators = {};
-
     let __gainNode = null;
+
     const getCtx = () => {
         if (typeof getAudioContext === 'function') {
             const ctx = getAudioContext();
@@ -33,23 +33,29 @@
 
     let _synthInstance = null;
     let _initialized = false;
+    let _initPromise = null;
 
     const SonicSF = {
         loadedFonts: {},
 
         init: async function (audioContext) {
             if (_initialized && _synthInstance) return;
-            console.log("[SonicSF] SpessaSynth init (oscillator fallback for preview).");
+            if (_initPromise) return _initPromise;
-            try {
+            _initPromise = (async () => {
-                const procUrl = (window.__SpessaSynthCDN || "https://cdn.jsdelivr.net/npm/spessasynth_lib@4.3.1/dist/") + "spessasynth_processor.min.js";
+                if (!window.SpessaSynthClass || !window.__SpessaSynthCDN) return;
-                await audioContext.audioWorklet.addModule(procUrl);
+                try {
-                _synthInstance = new (window.SpessaSynthClass)(audioContext);
+                    if (audioContext.state === 'suspended') await audioContext.resume();
-                _synthInstance.connect(audioContext.destination);
+                    const procUrl = window.__SpessaSynthCDN + "spessasynth_processor.min.js";
-                await _synthInstance.isReady;
+                    await audioContext.audioWorklet.addModule(procUrl);
-                _initialized = true;
+                    _synthInstance = new window.SpessaSynthClass(audioContext);
-            } catch (e) {
+                    _synthInstance.connect(audioContext.destination);
-                console.warn("[SonicSF] SpessaSynth init failed:", e);
+                    await _synthInstance.isReady;
-            }
+                    _initialized = true;
+                } catch (e) {
+                    console.warn("[SonicSF] SpessaSynth init failed:", e);
+                }
+            })();
+            return _initPromise;
         },
 
         loadSoundFont: async function (sfId) {
@@ -58,16 +64,15 @@
                 const cache = window.SonicSFStorage;
                 let buf = cache ? await cache.getBuffer(sfId) : null;
                 if (!buf) {
-                    const resp = await fetch("/api/v1/plugins/soundfonts/download/" + encodeURIComponent(sfId) + "?t=" + Date.now());
+                    const url = "/api/v1/plugins/soundfonts/download/" + encodeURIComponent(sfId) + "?t=" + Date.now();
+                    const resp = await fetch(url);
                     if (!resp.ok) return false;
                     buf = await resp.arrayBuffer();
                     if (cache) await cache.saveBuffer(sfId, buf);
                 }
                 await _synthInstance.soundBankManager.addSoundBank(buf.slice(0), sfId);
                 await _synthInstance.isReady;
-                const count = _synthInstance.soundBankManager?.selectablePresetList?.length ?? 0;
+                return true;
-                if (count === 0) console.warn("[SonicSF] SF parsed but 0 presets (SpessaSynth parser limitation). Using oscillator.");
-                return count > 0;
             } catch (e) {
                 console.warn("[SonicSF] loadSoundFont failed:", e);
                 return false;
@@ -75,8 +80,14 @@
         },
 
         selectInstrument: async function (channel, bank, program, sfId) {
-            if (!_initialized || !_synthInstance) return;
+            if (!_initialized || !_synthInstance) {
-            if (sfId) await this.loadSoundFont(sfId);
+                const ctx = new (window.AudioContext || window.webkitAudioContext)();
+                await this.init(ctx);
+            }
+            if (sfId) {
+                const ok = await this.loadSoundFont(sfId);
+                if (!ok) return;
+            }
             try { _synthInstance.controllerChange(channel, 0, bank); } catch (e) {}
             try { _synthInstance.controllerChange(channel, 32, 0); } catch (e) {}
             try { _synthInstance.programChange(channel, program); } catch (e) {}
@@ -126,11 +137,37 @@
         },
 
         playNote: function (note, velocity, durationMs, startTime, program, destinationNode, channel, synthEngine) {
-            // Always use oscillator for client preview — reliable
+            if (_initialized && _synthInstance) {
-            // Server-side FluidSynth render provides authentic SoundFont audio on Export
+                this._playNoteSpessa(note, velocity, durationMs, startTime, program, channel, synthEngine);
+                return;
+            }
             this._playNoteOsc(note, velocity, durationMs, startTime, program, destinationNode, channel, synthEngine);
         },
 
+        _playNoteSpessa: function (note, velocity, durationMs, startTime, program, channel, synthEngine) {
+            const ctx = getCtx();
+            const midiPitch = Math.min(127, Math.max(0, parseInt(note) || 60));
+            const midiVel = Math.min(127, Math.max(1, Math.floor(
+                typeof velocity === 'number' ? (velocity > 1 ? velocity : velocity * 127) : 100
+            )));
+            if (synthEngine) {
+                const ch = channel !== undefined ? channel : (synthEngine.soundfont_bank === 128 ? 9 : 0);
+                try { _synthInstance.controllerChange(ch, 0, synthEngine.soundfont_bank || 0); } catch (e) {}
+                try { _synthInstance.controllerChange(ch, 32, 0); } catch (e) {}
+                try { _synthInstance.programChange(ch, synthEngine.soundfont_program || 0); } catch (e) {}
+                if (channel === undefined) channel = ch;
+            }
+            if (channel === undefined) channel = 0;
+            const durSec = durationMs / 1000;
+            try {
+                _synthInstance.noteOn(channel, midiPitch, midiVel);
+                setTimeout(() => { try { _synthInstance.noteOff(channel, midiPitch); } catch (e) {} }, durSec * 1000);
+            } catch (e) {
+                console.warn("[SonicSF] SpessaSynth noteOn error, oscillator fallback:", e);
+                this._playNoteOsc(note, velocity, durationMs, startTime, program, null, channel, synthEngine);
+            }
+        },
+
         _playNoteOsc: function (note, velocity, durationMs, startTime, program, destinationNode, channel, synthEngine) {
             const ctx = getCtx();
             const freq = 440 * Math.pow(2, (note - 69) / 12);