feat: add privacy, terms, and cookies pages

Footer and login legal links were stubs. Point them at real marketing
pages that follow the Figma content-page template.

tests/unit/legalSyntheticPost.test.ts

@@ -0,0 +1,31 @@
+import { describe, expect, it } from "vitest";
+import {
+  buildLegalSyntheticPost,
+  legalPathForSlug,
+} from "../../lib/legalSyntheticPost";
+
+describe("legalSyntheticPost", () => {
+  it("maps slugs to public paths", () => {
+    expect(legalPathForSlug("privacy")).toBe("/privacy");
+    expect(legalPathForSlug("terms")).toBe("/terms");
+    expect(legalPathForSlug("cookies")).toBe("/cookies");
+  });
+
+  it("builds a guide-shaped post from page messages", () => {
+    const post = buildLegalSyntheticPost("privacy", {
+      banner: {
+        title: "Privacy Policy",
+        description: "How we handle information.",
+        author: "Media Economies Design Lab",
+        date: "2026-08-15",
+      },
+      updated: "Last updated August 15, 2026.",
+      intro: ["Hello."],
+      sections: [{ heading: "What we collect", paragraphs: ["Email."] }],
+    });
+
+    expect(post.slug).toBe("__legal__:privacy");
+    expect(post.frontmatter.title).toBe("Privacy Policy");
+    expect(post.filePath).toBe("messages/en/pages/privacy.json");
+  });
+});