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.

lib/legalSyntheticPost.ts

@@ -0,0 +1,53 @@
+import type { BlogPost } from "./content";
+
+export type LegalDocumentSlug = "privacy" | "terms" | "cookies";
+
+export type LegalDocumentSection = {
+  heading: string;
+  paragraphs: string[];
+};
+
+export type LegalDocumentMessages = {
+  banner: {
+    title: string;
+    description: string;
+    author: string;
+    date: string;
+  };
+  updated: string;
+  intro: string[];
+  sections: LegalDocumentSection[];
+};
+
+const PATH_BY_SLUG: Record<LegalDocumentSlug, string> = {
+  privacy: "/privacy",
+  terms: "/terms",
+  cookies: "/cookies",
+};
+
+export function legalPathForSlug(slug: LegalDocumentSlug): string {
+  return PATH_BY_SLUG[slug];
+}
+
+/**
+ * Builds a {@link BlogPost}-shaped object so legal pages can reuse
+ * `ContentBanner` (guide variant) without a markdown file.
+ */
+export function buildLegalSyntheticPost(
+  slug: LegalDocumentSlug,
+  page: LegalDocumentMessages,
+): BlogPost {
+  return {
+    slug: `__legal__:${slug}`,
+    frontmatter: {
+      title: page.banner.title,
+      description: page.banner.description,
+      author: page.banner.author,
+      date: page.banner.date,
+    },
+    content: "",
+    htmlContent: "",
+    filePath: `messages/en/pages/${slug}.json`,
+    lastModified: new Date(page.banner.date),
+  };
+}