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/post/[slug]/+page.server.ts

@@ -121,7 +121,7 @@ export const load: PageServerLoad = async ({ params, locals, url }) => {
     socialImage: postImageUrl ?? undefined,
     breadcrumbItems: [
       { href: '/', label: 'Start' },
-      { href: '/posts', label: 'Beiträge' },
+      { href: '/posts/', label: 'Beiträge' },
       { label: post.headline ?? post.linkName ?? post.slug ?? post._slug ?? slug },
     ],
   };