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/+layout.server.ts

@@ -1,4 +1,7 @@
 import type { LayoutServerLoad } from './$types';
+
+export const trailingSlash = 'always';
+
 import {
   getNavigationByKey,
   NavigationKeys,
@@ -71,7 +74,7 @@ export const load: LayoutServerLoad = async ({ locals }) => {
     for (const p of posts) {
       const label = p.linkName ?? p.headline ?? p.slug ?? p._slug ?? '';
       const urlSlug = normalizeSlug(p.slug ?? p._slug);
-      const canonical = urlSlug ? `/post/${encodeURIComponent(urlSlug)}` : '';
+      const canonical = urlSlug ? `/post/${encodeURIComponent(urlSlug)}/` : '';
       const keySlug = normalizeSlug(p.slug);
       const keyUnderscore = normalizeSlug(p._slug);
       if (keySlug) {
@@ -111,7 +114,8 @@ export const load: LayoutServerLoad = async ({ locals }) => {
               .split('/')
               .filter(Boolean)
               .map((seg) => encodeURIComponent(seg))
-              .join('/');
+              .join('/') +
+            '/';
       const href = postHref ?? pageHref;
       headerLinks.push({ href, label });
     }