feat(engine): fix SF2 path resolution, add synth_engine struct and fallback chain

- Fix SF2 path: search upload dir, system dir, static dir (backward compat)
- Add synth_engine struct parsing with flat field fallback
- Add 3-level fallback: selected SF -> default SF -> oscillator synth
- Write synth_engine in setTrackInstrument UI setters
- Pass synth_engine context to client SoundFontPlayer and AI gateway

app/static/js/services/aiGateway.js

@@ -218,6 +218,7 @@ Ví dụ: "Hãy chọn và copy từ bar 4 đến bar 12 của track 1 sau đó
   function buildAIPromptContext(dawState) {
     const tracks = (dawState.tracks || []).map(t => {
       const clips = t.clips && t.clips.length > 0 ? t.clips : (t.buffer ? [{ id: 'default_' + t.id, name: t.name, startTime: t.startTime || 0, duration: t.buffer.duration }] : []);
+      const se = t.synth_engine || null;
       return {
         id: t.id,
         name: t.name,
@@ -227,6 +228,7 @@ Ví dụ: "Hãy chọn và copy từ bar 4 đến bar 12 của track 1 sau đó
         solo: t.solo,
         volumeDb: t.volumeDb ?? 0,
         pan: t.pan ?? 0,
+        synth_engine: se ? { type: se.type, plugin_id: se.plugin_id, soundfont_bank: se.soundfont_bank, soundfont_program: se.soundfont_program, soundfont_id: se.soundfont_id } : undefined,
         clips: clips.map(c => ({ id: c.id, name: c.name, startTime: parseFloat((c.startTime || 0).toFixed(3)), duration: parseFloat((c.buffer ? c.buffer.duration : 0).toFixed(3)) }))
       };
     });

app/static/js/services/soundfontPlayer.js

@@ -57,7 +57,11 @@
             return ch;
         },
 
-        applyAITrackInstrument: function (bank, program) {
+        applyAITrackInstrument: function (bank, program, synthEngine) {
+            if (synthEngine) {
+                bank = bank !== undefined ? bank : (synthEngine.soundfont_bank || 0);
+                program = program !== undefined ? program : (synthEngine.soundfont_program || 0);
+            }
             const channel = this.allocateChannel(bank);
             this.controllerChange(channel, 0, bank);
             this.programChange(channel, program);
@@ -79,11 +83,20 @@
             return buffer;
         },
 
-        playNote: function (note, velocity, durationMs, startTime, program, destinationNode, channel) {
+        playNote: function (note, velocity, durationMs, startTime, program, destinationNode, channel, synthEngine) {
             const ctx = getCtx();
             const freq = 440 * Math.pow(2, (note - 69) / 12);
             if (freq <= 0 || isNaN(freq)) return null;
 
+            // Apply synth_engine state if provided
+            if (synthEngine) {
+                const ch = channel !== undefined ? channel : (synthEngine.soundfont_bank === 128 ? 9 : 0);
+                this.controllerChange(ch, 0, synthEngine.soundfont_bank || 0);
+                this.programChange(ch, synthEngine.soundfont_program || 0);
+                if (channel === undefined) channel = ch;
+                if (program === undefined) program = synthEngine.soundfont_program;
+            }
+
             const osc = ctx.createOscillator();
             const noteGain = ctx.createGain();