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/components/BlogOverview.svelte

@@ -108,8 +108,8 @@
   }
 
   function tagHref(tagSlug: string | null): string {
-    if (!tagSlug) return basePath;
-    return `${basePath}/tag/${encodeURIComponent(tagSlug)}`;
+    if (!tagSlug) return `${basePath}/`;
+    return `${basePath}/tag/${encodeURIComponent(tagSlug)}/`;
   }
 
   const totalCount = $derived(totalPosts);