fix: single Synth button opens instrument modal, fix init race condition

- Remove small Synth button (already done earlier)
- Large Synth button now calls openInstrumentSelector (instrument modal)
- Fix _initPromise stale cache: reset to null when init fails
- Module not loaded yet → retries on next select instead of stuck

app/static/js/services/soundfontPlayer.js

@@ -44,7 +44,11 @@
             if (_initialized && _synthInstance) return;
             if (_initPromise) return _initPromise;
             _initPromise = (async () => {
-                if (!window.SpessaSynthClass || !window.__SpessaSynthCDN) return;
+                if (!window.SpessaSynthClass || !window.__SpessaSynthCDN) {
+                    console.warn("[SonicSF] SpessaSynth CDN not available. Retrying on next select.");
+                    _initPromise = null;
+                    return;
+                }
                 try {
                     if (audioContext.state === 'suspended') await audioContext.resume();
                     const procUrl = window.__SpessaSynthCDN + "spessasynth_processor.min.js";
@@ -55,6 +59,7 @@
                     _initialized = true;
                 } catch (e) {
                     console.warn("[SonicSF] SpessaSynth init failed:", e);
+                    _initPromise = null;
                 }
             })();
             return _initPromise;