Problem:
The codebase supports both native gRPC and Connect-RPC protocols, but auth
service was using grpc.SetHeader() which only works for native gRPC. This
caused "failed to set grpc header" errors when using Connect-RPC clients
(browsers using nice-grpc-web).

Solution:
- Created HeaderCarrier pattern for protocol-agnostic header setting
- HeaderCarrier stores headers in context for Connect-RPC requests
- Falls back to grpc.SetHeader for native gRPC requests
- Updated auth service to use SetResponseHeader() instead of grpc.SetHeader()
- Refactored Connect wrappers to use withHeaderCarrier() helper to eliminate
  code duplication

Additional fixes:
- Allow public methods when gRPC metadata is missing in ACL interceptor
- Properly handle ParseSessionCookieValue errors instead of ignoring them
- Fix buildSessionCookie to gracefully handle missing metadata

Files changed:
- server/router/api/v1/header_carrier.go: New protocol-agnostic header carrier
- server/router/api/v1/auth_service.go: Use SetResponseHeader, handle missing metadata
- server/router/api/v1/connect_services.go: Use withHeaderCarrier helper
- server/router/api/v1/acl.go: Allow public methods without metadata
- server/router/api/v1/connect_interceptors.go: Handle ParseSessionCookieValue errors

🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>