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/routes/api/search-index/+server.ts

@@ -25,7 +25,7 @@ export type SearchHit = {
 function pageHref(p: { slug?: string; _slug?: string }): string {
   const raw = (p.slug ?? p._slug ?? '').replace(/^\//, '').trim();
   if (!raw || raw === 'home') return '/';
-  return '/' + raw.split('/').filter(Boolean).map(encodeURIComponent).join('/');
+  return '/' + raw.split('/').filter(Boolean).map(encodeURIComponent).join('/') + '/';
 }
 
 export const GET: RequestHandler = async () => {