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/products/[slug]/page.tsx

@@ -0,0 +1,39 @@
+import { notFound } from "next/navigation";
+import type { Metadata } from "next";
+import { getProductBySlug } from "@/services/product.service";
+import { BlockRenderer } from "@/blocks/BlockRenderer";
+
+interface ProductDetailPageProps {
+  params: Promise<{ slug: string }>;
+}
+
+export async function generateMetadata({
+  params,
+}: ProductDetailPageProps): Promise<Metadata> {
+  const { slug } = await params;
+  const product = await getProductBySlug(slug).catch(() => null);
+  return { title: product?.title ?? "产品未找到" };
+}
+
+export default async function ProductDetailPage({
+  params,
+}: ProductDetailPageProps) {
+  const { slug } = await params;
+  const product = await getProductBySlug(slug).catch(() => null);
+
+  if (!product) {
+    notFound();
+  }
+
+  return (
+    <article className="mx-auto max-w-3xl px-6 py-24">
+      <h1 className="mb-4 text-4xl font-bold text-slate-900">
+        {product.title}
+      </h1>
+      {product.summary && (
+        <p className="mb-12 text-lg text-slate-600">{product.summary}</p>
+      )}
+      <BlockRenderer blocks={product.body} />
+    </article>
+  );
+}