fix: use esm.sh CDN to resolve spessasynth_core bare import

- jdelivr raw index.js has bare import 'spessasynth_core' which browser can't resolve
- esm.sh automatically rewrites bare imports to full URLs
- Keep jdelivr for worklet processor (self-contained, no imports)
- WorkerSynthesizer fallback also uses esm.sh

app/static/js/services/soundfontPlayer.js

@@ -68,7 +68,8 @@
             } catch (e) {
                 console.warn("[SonicSF] SpessaSynth AudioWorklet init failed:", e, "- trying WorkerSynthesizer...");
                 try {
-                    const { WorkerSynthesizer } = await import(window.__SpessaSynthCDN + "index.js");
+                    const mod = await import("https://esm.sh/spessasynth_lib@4.3.1");
+                    const WorkerSynthesizer = mod.WorkerSynthesizer;
                     _synthInstance = new WorkerSynthesizer(audioContext);
                     await _synthInstance.isReady;
                     _initialized = true;