Add same-port support for Dev OAuth provider to fix SameSite cookie issues

Problem

The dev OAuth provider runs on a separate port (default 8084) from the main application. Since Chrome 80 (February 2020), browsers treat cookies without explicit SameSite attribute as SameSite=Lax by default. This breaks the dev OAuth flow because:

1. User visits 127.0.0.1:8080/auth/dev/login - JWT handshake cookie is set on port 8080
2. Browser redirects to 127.0.0.1:8084/login/oauth/authorize (dev OAuth server)
3. Dev OAuth redirects back to 127.0.0.1:8080/auth/dev/callback
4. Cookie is NOT sent because browsers treat different ports as different origins for SameSite purposes

Error: failed to get token - token cookie was not presented: http: named cookie not present

Why this happens

• Before 2020: Browsers sent cookies without SameSite restrictions
• After Chrome 80: Cookies default to SameSite=Lax, blocking cross-origin redirects
• Different ports (8080 vs 8084) = different origins from browser's cookie perspective
• Setting SameSite=None requires Secure=true which requires HTTPS (not practical for local dev)
• Setting SameSite=Lax or default doesn't help because modern browsers enforce Lax by default

Proposed Solution

Add an option to run the dev OAuth handler on the same port as the main application using path-based routing instead of port-based separation.

API Changes

1. Add OAuthBasePath field to Params:

type Params struct {
    // ... existing fields
    OAuthBasePath string // e.g., "/auth/dev/oauth" for same-port mode
}

2. Add Handler() method to DevAuthServer:

// Handler returns http.Handler for mounting on external router
func (d *DevAuthServer) Handler() http.Handler

3. Add DevAuthHandler() to Service:

// DevAuthHandler returns handler for same-port mounting
func (s *Service) DevAuthHandler() (http.Handler, error)

Usage (same-port mode)

// Configure with OAuthBasePath
authenticator.AddDevProvider("", 0, "/auth/dev/oauth")

// Get handler and mount on main router
devHandler, _ := authenticator.DevAuthHandler()
router.Mount("/auth/dev/oauth", devHandler)

Backward Compatibility

• Default behavior unchanged (separate port 8084)
• Same-port mode is opt-in via OAuthBasePath
• No breaking changes

References

• Chromium SameSite Updates
• SameSite cookies explained
• Related issue in remark42: dev login broken with modern browsers

Start by locating Params, DevAuthServer.Handler, Service.DevAuthHandler, and AddDevProvider. Trace how the existing separate-port handler is created and how the main router mounts handlers. Done means OAuthBasePath enables same-port mounting while the default separate-port behavior remains unchanged.