Add registration flow and improve admin post management

app/register/page.tsx

@@ -0,0 +1,23 @@
+import RegisterForm from "./register-form";
+
+export const metadata = {
+  title: "注册 - OPC Solo Feed"
+};
+
+export default function RegisterPage() {
+  return (
+    <div className="mx-auto max-w-md rounded-2xl bg-white/90 p-8 shadow-sm ring-1 ring-slate-100">
+      <h1 className="text-2xl font-semibold text-slate-900">注册账号</h1>
+      <p className="mt-2 text-sm text-slate-600">注册后 24 小时内有效，过期需重新登录。</p>
+      <div className="mt-6">
+        <RegisterForm />
+      </div>
+      <p className="mt-4 text-sm text-slate-500">
+        已有账号？{" "}
+        <a href="/login" className="text-brand-600 hover:text-brand-700">
+          去登录
+        </a>
+      </p>
+    </div>
+  );
+}