Fix OIDC login-gate cross-file wiring bugs from whole-branch review

Four bugs slipped through per-task review since each task only saw its
own diff:

- Logout was a plain <a href> GET against a POST-only backend route, so
  it 405'd and never cleared the session cookie or hit Keycloak's
  end-session redirect. Now a <form method="post"> with a submit button
  styled to match the old link (still a real full-page navigation, not
  a fetch, so the Keycloak redirect chain still works).
- Login/logout used origin-relative paths, unreachable from the Vite
  dev server (:5173) against the backend (:8080) with no proxy
  configured. Both now build their URL from client.ts's now-exported
  BASE_URL.
- handleSessionCallback's four failure paths redirected to
  /?auth_error=failed with no logging, making a real OIDC failure
  undiagnosable in production. Added log.Printf on each failure site.
- handleSessionLogout passed a bare "/" to EndSessionURL; Keycloak
  requires post_logout_redirect_uri to be an absolute, registered URL.
  Added SessionConfig.PublicBaseURL, wired from cfg.PublicBaseURL in
  main.go, and used to build an absolute redirect.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/cmd/geniusrund/main.go

@@ -61,9 +61,10 @@ func main() {
 	}
 
 	server := api.NewServer(db, garminClient, syncSvc, authVerifier, api.SessionConfig{
-		Secret:   cfg.SessionSecret,
-		Duration: cfg.SessionDuration,
-		Secure:   cfg.SessionSecure,
+		Secret:        cfg.SessionSecret,
+		Duration:      cfg.SessionDuration,
+		Secure:        cfg.SessionSecure,
+		PublicBaseURL: cfg.PublicBaseURL,
 	})
 
 	ctx, stop := signal.NotifyContext(context.Background(), syscall.SIGINT, syscall.SIGTERM)