Add category toggle-button filters to the blog overview

URL-search-param-driven (?categories=A,B), same shareable/bookmarkable
approach as the order-list filters — plain server-rendered toggle
Links, no client component needed since /blog already fetches every
published post in one request (limit 100, no pagination).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app/blog/page.tsx

@@ -20,8 +20,33 @@ export const metadata: Metadata = {
   },
 };
 
-export default async function BlogOverviewPage() {
-  const posts = await getBlogPosts(100);
+// Every distinct category name in DOM order — Payload's `categories` field
+// stores related docs, no separate slug on this side, but since this page
+// already fetches every published post (limit 100, no pagination), a
+// plain client-agnostic array filter is enough; no separate Payload query
+// per category needed.
+function distinctCategories(posts: { categories: string[] }[]): string[] {
+  const seen = new Set<string>();
+  for (const post of posts) for (const c of post.categories) seen.add(c);
+  return Array.from(seen);
+}
+
+function buildCategoryHref(active: string[], category: string): string {
+  const next = active.includes(category) ? active.filter((c) => c !== category) : [...active, category];
+  return next.length > 0 ? `/blog?categories=${next.map(encodeURIComponent).join(",")}` : "/blog";
+}
+
+export default async function BlogOverviewPage({
+  searchParams,
+}: {
+  searchParams: Promise<{ categories?: string }>;
+}) {
+  const allPosts = await getBlogPosts(100);
+  const { categories: categoriesParam } = await searchParams;
+  const activeCategories = categoriesParam ? categoriesParam.split(",").filter(Boolean) : [];
+  const allCategories = distinctCategories(allPosts);
+  const posts =
+    activeCategories.length === 0 ? allPosts : allPosts.filter((post) => post.categories.some((c) => activeCategories.includes(c)));
   const [featured, ...rest] = posts;
 
   return (
@@ -49,6 +74,41 @@ export default async function BlogOverviewPage() {
           </div>
         </Reveal>
 
+        {allCategories.length > 1 && (
+          <Reveal className="flex flex-wrap gap-2 w-full max-w-[80rem] mx-auto px-[var(--layout-padding-x)] pt-6">
+            {allCategories.map((category) => {
+              const active = activeCategories.includes(category);
+              return (
+                <Link
+                  key={category}
+                  href={buildCategoryHref(activeCategories, category)}
+                  className={`inline-flex items-center px-3 py-1.5 rounded-full text-body-sm font-semibold whitespace-nowrap border transition-colors ${
+                    active
+                      ? "bg-brand border-brand text-text-primary"
+                      : "bg-bg-base border-border text-text-muted hover:border-brand hover:text-brand"
+                  }`}
+                >
+                  {category}
+                </Link>
+              );
+            })}
+            {activeCategories.length > 0 && (
+              <Link
+                href="/blog"
+                className="inline-flex items-center px-3 py-1.5 text-body-sm font-semibold text-text-muted underline hover:text-brand transition-colors"
+              >
+                Zurücksetzen
+              </Link>
+            )}
+          </Reveal>
+        )}
+
+        {posts.length === 0 && (
+          <p className="text-body text-text-muted w-full max-w-[80rem] mx-auto px-[var(--layout-padding-x)] pt-10">
+            Keine Beiträge in dieser Kategorie gefunden.
+          </p>
+        )}
+
         {featured && (
           // -mt-8, not pt-10 — pulls the card up to slightly overlap the
           // hero section's bottom edge instead of sitting flush below it.