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>

frontend/src/LoginGate.tsx

@@ -1,5 +1,5 @@
 import { useEffect, useState } from "react";
-import { api } from "./api/client";
+import { api, BASE_URL } from "./api/client";
 import "./LoginGate.css";
 import App from "./App";
 import type { SessionInfo } from "./types/api";
@@ -39,7 +39,7 @@ export function LoginGate() {
       <div className="login-gate">
         <h1>🧞‍♀️ geniusrun</h1>
         {authError && <p className="login-gate-error">{AUTH_ERROR_MESSAGES[authError] ?? "Login failed, please try again."}</p>}
-        <a className="login-gate-button" href="/api/session/login">
+        <a className="login-gate-button" href={`${BASE_URL}/api/session/login`}>
           Log in
         </a>
       </div>