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/App.css

@@ -94,9 +94,14 @@ body {
 }
 
 .logout-link {
+  background: none;
+  border: none;
+  padding: 0;
+  font: inherit;
   color: #9aa0ab;
   font-size: 0.85rem;
   text-decoration: none;
+  cursor: pointer;
 }
 
 .logout-link:hover {

frontend/src/App.tsx

@@ -1,5 +1,5 @@
 import { useEffect, useState } from "react";
-import { api } from "./api/client";
+import { api, BASE_URL } from "./api/client";
 import "./App.css";
 import { Dashboard } from "./pages/Dashboard";
 import { Plan } from "./pages/Plan";
@@ -56,9 +56,11 @@ function App({ session }: { session: SessionInfo }) {
           >
             {profileName ?? "Profile"}
           </button>
-          <a className="logout-link" href="/api/session/logout" title={session.email}>
-            Log out
-          </a>
+          <form method="post" action={`${BASE_URL}/api/session/logout`} title={session.email}>
+            <button type="submit" className="logout-link">
+              Log out
+            </button>
+          </form>
         </div>
       </header>
       <main>{showProfile ? <Profile onSaved={(p) => setProfileName(p.Name)} /> : <Active />}</main>

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>

frontend/src/api/client.ts

@@ -13,7 +13,7 @@ import type {
   WorkoutKind,
 } from "../types/api";
 
-const BASE_URL = import.meta.env.VITE_API_BASE_URL ?? "http://localhost:8080";
+export const BASE_URL = import.meta.env.VITE_API_BASE_URL ?? "http://localhost:8080";
 
 async function request<T>(path: string, init?: RequestInit): Promise<T> {
   const res = await fetch(`${BASE_URL}${path}`, {
@@ -39,9 +39,12 @@ async function request<T>(path: string, init?: RequestInit): Promise<T> {
 
 export const api = {
   // Session (app login via OIDC -- distinct from the Garmin credential
-  // login below). No logout()/login() methods: those are plain <a href>
-  // full-page navigations (see LoginGate.tsx), not fetches, since the OIDC
-  // flow and Keycloak's own logout redirect need real browser navigation.
+  // login below). No logout()/login() methods: login is a plain <a href>
+  // and logout is a <form method="post"> submit button (see App.tsx /
+  // LoginGate.tsx) -- both are real full-page navigations, not fetches,
+  // since the OIDC flow and Keycloak's own logout redirect need real
+  // browser navigation. Logout must POST (see server.go's route), which an
+  // <a href> can't do, hence the form.
   getSessionInfo: () => request<SessionInfo>("/api/session/me"),
 
   // Auth