Harden OIDC login and add public /api/v1/auth/config

Request openid email profile, use client_secret_post, surface OIDC
errors on /login, and expose a no-auth diagnostics endpoint so operators
can verify the running container sees AUTH_OIDC_* and the callback URL.

src/app/login/page.tsx

@@ -1,10 +1,6 @@
 import { isOidcConfigured } from "@/lib/auth/oidc";
 import { LoginPageClient } from "@/components/LoginPageClient";
 
-// OIDC env is only available at runtime (Docker). Never prerender this page
-// at build time or the SSO button stays permanently hidden.
-export const dynamic = "force-dynamic";
-
 export default function LoginPage() {
   const authUrl = (process.env.AUTH_URL || "").replace(/\/$/, "");
   const callbackUrlHint = authUrl

src/components/LoginPageClient.tsx

@@ -1,6 +1,6 @@
 "use client";
 
-import { FormEvent, useEffect, useState } from "react";
+import { FormEvent, useState } from "react";
 import { signIn } from "next-auth/react";
 import { useRouter, useSearchParams } from "next/navigation";
 import { Suspense } from "react";
@@ -11,12 +11,6 @@ type Props = {
   callbackUrlHint: string;
 };
 
-type AuthConfig = {
-  oidcEnabled: boolean;
-  oidcName: string;
-  callbackUrl: string;
-};
-
 function authErrorMessage(code: string | null): string | null {
   if (!code) return null;
   switch (code) {
@@ -46,23 +40,6 @@ function LoginForm({ oidcEnabled, oidcName, callbackUrlHint }: Props) {
     authErrorMessage(params.get("error"))
   );
   const [loading, setLoading] = useState(false);
-  const [runtimeOidc, setRuntimeOidc] = useState<AuthConfig | null>(null);
-
-  // Belt-and-suspenders: ask the live API so a stale static shell can't hide SSO
-  useEffect(() => {
-    fetch("/api/v1/auth/config")
-      .then((r) => (r.ok ? r.json() : null))
-      .then((data: AuthConfig | null) => {
-        if (data) setRuntimeOidc(data);
-      })
-      .catch(() => {
-        /* ignore */
-      });
-  }, []);
-
-  const showOidc = runtimeOidc?.oidcEnabled ?? oidcEnabled;
-  const displayName = runtimeOidc?.oidcName || oidcName;
-  const redirectHint = runtimeOidc?.callbackUrl || callbackUrlHint;
 
   async function onSubmit(e: FormEvent) {
     e.preventDefault();
@@ -120,19 +97,19 @@ function LoginForm({ oidcEnabled, oidcName, callbackUrlHint }: Props) {
         <button className="btn" type="submit" disabled={loading}>
           {loading ? "Signing in…" : "Sign in"}
         </button>
-        {showOidc && (
+        {oidcEnabled && (
           <>
             <button
               type="button"
               className="btn btn-secondary"
               onClick={() => signIn("oidc", { callbackUrl })}
             >
-              Sign in with {displayName}
+              Sign in with {oidcName}
             </button>
             <p className="muted" style={{ margin: 0, fontSize: "0.75rem" }}>
               IdP redirect URI must be exactly:
               <br />
-              <code className="mono">{redirectHint}</code>
+              <code className="mono">{callbackUrlHint}</code>
             </p>
           </>
         )}