seo: enforce trailing slash on all URLs

- Set trailingSlash='always' globally via root layout.
- Update postHref, pageHref, Pagination, tag/breadcrumb hrefs, blog→posts
  redirects to emit trailing slash.
- Match old Astro canonical so indexed URLs hit 200 directly instead of
  308-redirect.

src/lib/blog-utils.ts

@@ -302,7 +302,7 @@ const POST_BASE = "/post";
 export function postHref(post: PostEntry): string {
   const slug = post.slug ?? post._slug ?? "";
   const norm = (slug ?? "").replace(/^\//, "").trim();
-  return norm ? `${POST_BASE}/${encodeURIComponent(norm)}` : POST_BASE;
+  return norm ? `${POST_BASE}/${encodeURIComponent(norm)}/` : `${POST_BASE}/`;
 }
 
 export function formatPostDate(value: string | undefined): string {