Configure API requests for reverse proxy setup

- Updated all API endpoints to use /rechner/api/ path
- Removed CSP localhost restriction for reverse proxy compatibility
- Added nginx proxy configuration for API requests
- Should now work through HTTPS reverse proxy

html/rechner/index.html

@@ -8,7 +8,7 @@
     script-src 'self' 'unsafe-inline' 'unsafe-eval' https://cdn.jsdelivr.net;
     style-src 'self' 'unsafe-inline';
     img-src 'self' data:;
-    connect-src 'self' http://localhost:3001;
+    connect-src 'self';
   ">
   <title>Rechner</title>