fix: resolve JSON.parse error in frontend API calls

- Add error handling for JSON encoding in all backend handlers
- Add CORS support to backend server
- Add proxy configuration for frontend development server
- Improve error handling in frontend API calls
- Build frontend to create static files for backend serving
- Add comprehensive error messages and user feedback

Fixes issue where frontend received malformed JSON responses from backend API.

backend/go.mod

@@ -10,6 +10,7 @@ require (
 
 require (
 	github.com/inconshreveable/mousetrap v1.1.0 // indirect
+	github.com/rs/cors v1.11.1 // indirect
 	github.com/spf13/pflag v1.0.5 // indirect
 	golang.org/x/sys v0.0.0-20220715151400-c0bba94af5f8 // indirect
 )