fix: keep all banks loaded, don't delete on switch

- Remove deleteSoundBank — SpessaSynth merges presets from all banks
- Check soundBankList for existing bank before re-downloading
- Handle 404 gracefully — warn but don't break subsequent loads
- Failed load doesn't delete previously loaded banks

app/static/js/services/soundfontPlayer.js

@@ -63,17 +63,22 @@
         loadSoundFont: async function (sfId) {
             if (!_initialized || !_synthInstance) return false;
             if (_currentSfId === sfId) return true;
+            // Check if already loaded (SpessaSynth can hold multiple banks)
+            const already = _synthInstance.soundBankManager?.soundBankList?.some(b => b.id === sfId);
+            if (already) {
+                _currentSfId = sfId;
+                return true;
+            }
             try {
-                // Unload previous SoundFont
-                if (_currentSfId) {
-                    try { _synthInstance.soundBankManager.deleteSoundBank(_currentSfId); } catch (e) {}
-                }
                 const cache = window.SonicSFStorage;
                 let buf = cache ? await cache.getBuffer(sfId) : null;
                 if (!buf) {
                     const url = "/api/v1/plugins/soundfonts/download/" + encodeURIComponent(sfId) + "?t=" + Date.now();
                     const resp = await fetch(url);
-                    if (!resp.ok) return false;
+                    if (!resp.ok) {
+                        console.warn("[SonicSF] SoundFont not found on server:", sfId);
+                        return false;
+                    }
                     buf = await resp.arrayBuffer();
                     if (cache) await cache.saveBuffer(sfId, buf);
                 }