Navigating from login reused a send_email_code token on register.
Captcha failure used 401 so the client stored /register as the
post-login target and never left the page. Permission denials now
return 403, and the API client no longer wipes the session on 401.
- Relocated go.mod and go.sum into backend/ root directory
- Stripped redundant backend/ segments from all Go imports (github.com/Rain-kl/Wavelet/...)
- Unified Makefile, swagger, and build-test to execute in backend/ module context
- Ensured 100% build-test, code-check, format, and swagger pass
ryan