Add featured-post flag, align tool-page connector arrows, misc blog polish

- Posts collection: new "featured" checkbox (Payload) — /blog now prefers
  a manually-featured post over just the most recent one. Query sorts by
  "-featured,-publishedAt" so multiple accidentally-featured posts
  resolve deterministically (most recent among them wins) instead of
  erroring or being ambiguous.
- challenge/page.tsx: step-connector arrow now uses the same
  /icon-arrow-connector.svg asset as todo-cards/newsletter's HowItWorks,
  replacing its own hand-drawn inline SVG.
- todo-cards & newsletter HowItWorks: connector arrows now vertically
  centered on their icon (md:mt-[1.625rem]), matching Challenge's
  margin-based centering technique instead of sitting flush at the row's
  top edge.
- Blog overview: featured card overlaps the header slightly (-mt-8) per
  Figma-inspired refinement, and now also shows the publish date in its
  meta row (previously only the list items did).

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

app/lib/payload.ts

@@ -10,6 +10,7 @@ export type BlogPost = {
   excerpt: string;
   thumbnail: string | null;
   publishedAt: string;
+  featured: boolean;
 };
 
 type PayloadPost = {
@@ -21,12 +22,18 @@ type PayloadPost = {
   excerpt: string;
   thumbnail: { url: string } | number | null;
   publishedAt: string;
+  featured: boolean;
 };
 
+// sort: "-featured,-publishedAt" — Payload's multi-field sort puts any
+// featured post(s) first, most-recently-published first among those, then
+// the rest by date. Callers that just want posts[0] as "the" featured post
+// (e.g. /blog) get a deterministic pick even if more than one post was
+// accidentally marked featured — no special-casing needed here.
 export async function getBlogPosts(limit = 3): Promise<BlogPost[]> {
   const params = new URLSearchParams({
     "where[tenant.slug][equals]": TENANT_SLUG,
-    sort: "-publishedAt",
+    sort: "-featured,-publishedAt",
     depth: "2",
     limit: String(limit),
   });
@@ -56,6 +63,7 @@ export async function getBlogPosts(limit = 3): Promise<BlogPost[]> {
         ? post.thumbnail.url
         : null,
     publishedAt: post.publishedAt,
+    featured: post.featured,
   }));
 }
 
@@ -96,6 +104,7 @@ export async function getPostBySlug(slug: string): Promise<PostDetail | null> {
       typeof doc.thumbnail === "object" && doc.thumbnail ? doc.thumbnail.url : null,
     content: doc.content,
     publishedAt: doc.publishedAt,
+    featured: doc.featured,
   };
 }