fix: gracefully fallback to oscillator when SpessaSynth bank load fails

- SpessaSynth init tries to load default bank, gracefully handles failure
- When bank load fails, _currentSfId stays null → _playNoteOsc fallback
- Client preview: oscillator emulation (approximate GM sound)
- Server-side render via FluidSynth: authentic SoundFont audio

app/static/js/services/soundfontPlayer.js

@@ -60,16 +60,46 @@
                 await audioContext.audioWorklet.addModule(procUrl);
                 _synthInstance = new window.SpessaSynthClass(audioContext);
                 await _synthInstance.isReady;
+                // Load default SF2 to verify parsing works
+                try {
+                    const resp = await fetch("/api/v1/plugins/soundfonts/download/sgm_v2.01?t=" + Date.now());
+                    if (resp.ok) {
+                        const buf = await resp.arrayBuffer();
+                        await _synthInstance.soundBankManager.addSoundBank(buf.slice(0), "sgm_v2.01");
+                        await _synthInstance.isReady;
+                        // Check if presets were loaded by querying soundBankList
+                        const bankCount = _synthInstance.soundBankManager?.soundBankList?.length ?? 0;
+                        console.log("[SonicSF] SpessaSynth initialized. Banks loaded:", bankCount);
+                        if (bankCount > 0) {
+                            _initialized = true;
+                            _initInProgress = false;
+                            return;
+                        }
+                    }
+                } catch (bankErr) {
+                    console.warn("[SonicSF] Default bank load failed:", bankErr);
+                }
+                // Fallback: init without bank — oscillator will handle preview
                 _initialized = true;
                 _initInProgress = false;
-                console.log("[SonicSF] SpessaSynth initialized via AudioWorklet.");
+                console.log("[SonicSF] SpessaSynth initialized (no sound bank). Oscillator fallback active.");
             } catch (e) {
                 _initInProgress = false;
-                console.warn("[SonicSF] AudioWorklet failed, trying WorkerSynthesizer:", e);
+                console.warn("[SonicSF] AudioWorklet failed:", e);
+                // Try WorkerSynthesizer as fallback
                 try {
                     const mod = await import("https://cdn.jsdelivr.net/npm/spessasynth_lib@4.3.1/dist/index.js");
                     _synthInstance = new mod.WorkerSynthesizer(audioContext);
                     await _synthInstance.isReady;
+                    try {
+                        const resp = await fetch("/api/v1/plugins/soundfonts/download/sgm_v2.01?t=" + Date.now());
+                        if (resp.ok) {
+                            const buf = await resp.arrayBuffer();
+                            await _synthInstance.soundBankManager.addSoundBank(buf.slice(0), "sgm_v2.01");
+                            await _synthInstance.isReady;
+                            console.log("[SonicSF] Worker synth with bank OK.");
+                        }
+                    } catch (e2) {}
                     _initialized = true;
                     console.log("[SonicSF] SpessaSynth initialized via Worker.");
                 } catch (e2) {
@@ -77,6 +107,7 @@
                 }
             }
         },
+        },
 
         // ── Load SF3 from IndexedDB cache or server ──
         loadSoundFont: async function (sfId) {