Add blog overview page (/blog), link Navbar "Blog" to it

- app/blog/page.tsx: header (title/subheading + bleeding photo with a
  left-edge fade, per Figma node 4577:330), featured-post card (most
  recent), remaining posts as a divided list with category/readTime/date,
  and the reusable Newsletter signup panel. Uses all real posts from
  Payload — the Figma mockup padded its list out to 4 items with two
  posts that were never actually seeded, so this only renders what
  exists (currently 1 featured + 2 listed).
- lib/payload.ts: BlogPost now carries publishedAt (needed for the list's
  per-post date), removed the now-redundant duplicate field on PostDetail.
- Navbar: "Blog" now links to /blog instead of scrolling to the
  homepage's #blog anchor — same real-route pattern as "Shop".
- blog/[slug]/page.tsx: earlier title-size tuning (fluid clamp between
  text-h-feature and text-display) from this session's follow-up
  feedback, not yet pushed.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app/lib/payload.ts

@@ -9,6 +9,7 @@ export type BlogPost = {
   readTime: number;
   excerpt: string;
   thumbnail: string | null;
+  publishedAt: string;
 };
 
 type PayloadPost = {
@@ -19,6 +20,7 @@ type PayloadPost = {
   readTime: number;
   excerpt: string;
   thumbnail: { url: string } | number | null;
+  publishedAt: string;
 };
 
 export async function getBlogPosts(limit = 3): Promise<BlogPost[]> {
@@ -53,15 +55,15 @@ export async function getBlogPosts(limit = 3): Promise<BlogPost[]> {
       typeof post.thumbnail === "object" && post.thumbnail
         ? post.thumbnail.url
         : null,
+    publishedAt: post.publishedAt,
   }));
 }
 
 export type PostDetail = BlogPost & {
   content: unknown;
-  publishedAt: string;
 };
 
-type PayloadPostDetail = PayloadPost & { content: unknown; publishedAt: string };
+type PayloadPostDetail = PayloadPost & { content: unknown };
 
 export async function getPostBySlug(slug: string): Promise<PostDetail | null> {
   const params = new URLSearchParams({