fix: serve SpessaSynth processor locally to avoid CDN version mismatch

- Download spessasynth_processor.min.js from CDN, serve as static file
- Change __SpessaSynthCDN to /static/js/services/
- Fix version mismatch between esm.sh library and CDN processor

app/templates/index.html

@@ -13,7 +13,7 @@
     <script type="module">
       import { WorkletSynthesizer } from 'https://esm.sh/spessasynth_lib@4.3.1';
       window.SpessaSynthClass = WorkletSynthesizer;
-      window.__SpessaSynthCDN = 'https://cdn.jsdelivr.net/npm/spessasynth_lib@4.3.1/dist/';
+      window.__SpessaSynthCDN = '/static/js/services/';
       console.log('[SonicSF] SpessaSynth library loaded via esm.sh.');
     </script>
     <script src="/static/js/services/api.js?v=202607232105"></script>