feat: add products/cases pages and lead-capture form block

- types: Product/CaseStudy page types, CaseStudyBlockValue, FormBlockValue
- services: product.service.ts, case.service.ts
- app/products, app/cases: list + detail pages
- blocks: CaseStudyCard, LeadForm (submits to /api/v1/custom/leads/)
- Header nav: add 产品/案例 links

blocks/case-study/CaseStudyCard.tsx

@@ -0,0 +1,24 @@
+import Link from "next/link";
+import type { CaseStudyBlockValue } from "@/types/wagtail";
+
+export default function CaseStudyCard({ value }: { value: CaseStudyBlockValue }) {
+  if (!value.case_page) {
+    return null;
+  }
+
+  return (
+    <div className="mx-auto max-w-3xl px-6 py-8">
+      <Link
+        href={value.case_page.meta.html_url ?? "#"}
+        className="block rounded-3xl border border-slate-100 p-8 shadow-sm transition hover:shadow-md"
+      >
+        <h3 className="text-xl font-semibold text-slate-900">
+          {value.case_page.title}
+        </h3>
+        {value.summary && (
+          <p className="mt-2 text-slate-600">{value.summary}</p>
+        )}
+      </Link>
+    </div>
+  );
+}