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

app/cases/[slug]/page.tsx

@@ -0,0 +1,38 @@
+import { notFound } from "next/navigation";
+import type { Metadata } from "next";
+import { getCaseStudyBySlug } from "@/services/case.service";
+import { BlockRenderer } from "@/blocks/BlockRenderer";
+
+interface CaseStudyDetailPageProps {
+  params: Promise<{ slug: string }>;
+}
+
+export async function generateMetadata({
+  params,
+}: CaseStudyDetailPageProps): Promise<Metadata> {
+  const { slug } = await params;
+  const item = await getCaseStudyBySlug(slug).catch(() => null);
+  return { title: item?.title ?? "案例未找到" };
+}
+
+export default async function CaseStudyDetailPage({
+  params,
+}: CaseStudyDetailPageProps) {
+  const { slug } = await params;
+  const item = await getCaseStudyBySlug(slug).catch(() => null);
+
+  if (!item) {
+    notFound();
+  }
+
+  return (
+    <article className="mx-auto max-w-3xl px-6 py-24">
+      <h1 className="mb-2 text-4xl font-bold text-slate-900">{item.title}</h1>
+      <p className="mb-12 text-sm text-slate-500">
+        {item.client_name}
+        {item.industry ? ` · ${item.industry}` : ""}
+      </p>
+      <BlockRenderer blocks={item.body} />
+    </article>
+  );
+}