feat(platform): add Discord onboarding and Velocity gate

apps/web/src/app/auth/error/page.tsx

@@ -0,0 +1,14 @@
+import Link from "next/link";
+
+export default function LoginErrorPage() {
+  return (
+    <main className="grid min-h-screen place-items-center bg-canvas px-6 text-ink">
+      <section className="max-w-lg border border-line bg-panel p-8 shadow-[10px_10px_0_var(--color-shadow)]">
+        <p className="font-mono text-xs font-bold uppercase tracking-[0.25em] text-accent">Link rejected</p>
+        <h1 className="mt-4 font-display text-4xl font-black uppercase tracking-tight">That gate key no longer works.</h1>
+        <p className="mt-5 leading-7 text-muted">Login links expire after ten minutes and can only be used once. Return to Discord and run <strong>/account</strong> for a fresh link.</p>
+        <Link className="mt-7 inline-block border border-ink bg-ink px-5 py-3 font-mono text-xs font-bold uppercase tracking-wider text-canvas" href="/">Back home</Link>
+      </section>
+    </main>
+  );
+}