feat(auth): hide Google sign-in until OAuth is configured

Google social login has no credentials in production, so the "Continue with
Google" button (and its divider) errored on click. Gate the button + divider on
a new isGoogleConfigured() helper (requires GOOGLE_CLIENT_ID + GOOGLE_CLIENT_SECRET)
on both the login and signup pages, and guard the signInWithGoogle action as
defense in depth. The button reappears automatically once both env vars are set.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/actions/auth.ts

@@ -3,7 +3,7 @@
 import { redirect } from "next/navigation"
 import { headers } from "next/headers"
 import { APIError } from "better-auth/api"
-import { auth } from "@/lib/auth"
+import { auth, isGoogleConfigured } from "@/lib/auth"
 import { verifyTurnstile } from "@/lib/turnstile"
 
 const APP_URL = process.env.NEXT_PUBLIC_APP_URL ?? "http://localhost:3000"
@@ -77,6 +77,11 @@ export async function signIn(formData: FormData) {
 }
 
 export async function signInWithGoogle() {
+  // Defense in depth: the auth pages hide the Google button when it isn't
+  // configured, but guard the action too in case it's POSTed directly.
+  if (!isGoogleConfigured()) {
+    redirect(`/login?error=${encodeURIComponent("Google sign-in isn't available right now.")}`)
+  }
   let url: string | undefined
   try {
     const res = await auth.api.signInSocial({