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/internal/api/api_test.go

@@ -23,9 +23,10 @@ import (
 func newCtx() context.Context { return context.Background() }
 
 var testSessionConfig = SessionConfig{
-	Secret:   []byte("test-session-secret-at-least-32-bytes-long"),
-	Duration: time.Hour,
-	Secure:   false,
+	Secret:        []byte("test-session-secret-at-least-32-bytes-long"),
+	Duration:      time.Hour,
+	Secure:        false,
+	PublicBaseURL: "https://geniusrun.example.com",
 }
 
 func newTestServer(t *testing.T) (*Server, *store.DB) {