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

services/product.service.ts

@@ -0,0 +1,26 @@
+import { fetchAPI } from "./api-client";
+import type {
+  ProductPageDetail,
+  ProductPageSummary,
+  WagtailListResponse,
+} from "@/types/wagtail";
+
+/** 获取产品列表。 */
+export async function getProductList(): Promise<ProductPageSummary[]> {
+  const res = await fetchAPI<WagtailListResponse<ProductPageSummary>>(
+    "/api/v2/pages/?type=products.ProductPage&fields=summary",
+    { tags: ["product-list"] },
+  );
+  return res.items;
+}
+
+/** 根据 slug 获取产品详情。 */
+export async function getProductBySlug(
+  slug: string,
+): Promise<ProductPageDetail | null> {
+  const res = await fetchAPI<WagtailListResponse<ProductPageDetail>>(
+    `/api/v2/pages/?type=products.ProductPage&slug=${encodeURIComponent(slug)}&fields=body,summary&limit=1`,
+    { tags: [`product:${slug}`] },
+  );
+  return res.items[0] ?? null;
+}