Refactor navigation links to use Next.js routing and improve post handling

- Updated AboutButton to navigate to the about page using Next.js router.
- Changed HeaderButtons and MobileNav to link directly to the about page.
- Modified Home component to exclude the 'about' post from the posts list.
- Added a helper function to strip YAML frontmatter from post summaries.
- Enhanced API routes to handle reading and writing markdown files for posts.

src/app/page.tsx

@@ -148,11 +148,21 @@ export default function Home() {
     })),
   ];
 
+  // Helper to strip YAML frontmatter
+  function stripFrontmatter(md: string): string {
+    if (!md) return '';
+    if (md.startsWith('---')) {
+      const end = md.indexOf('---', 3);
+      if (end !== -1) return md.slice(end + 3).replace(/^\s+/, '');
+    }
+    return md;
+  }
+
   // Helper to recursively collect all posts from the tree
   function collectPosts(nodes: Node[]): Post[] {
     let posts: Post[] = [];
     for (const node of nodes) {
-      if (node.type === 'post') {
+      if (node.type === 'post' && node.slug !== 'about') {
         posts.push(node);
       } else if (node.type === 'folder') {
         posts = posts.concat(collectPosts(node.children));
@@ -258,7 +268,7 @@ export default function Home() {
                         )}
                         <div>Erstellt: {format(new Date(post.createdAt), 'd. MMMM yyyy HH:mm')}</div>
                       </div>
-                      <p className="text-gray-700 mb-3 sm:mb-4 text-sm sm:text-base">{post.summary}</p>
+                      <p className="text-gray-700 mb-3 sm:mb-4 text-sm sm:text-base">{stripFrontmatter(post.summary)}</p>
                       <div className="flex flex-wrap gap-1 sm:gap-2">
                         {post.tags.map((tag: string) => {
                           const q = search.trim().toLowerCase();
@@ -317,7 +327,7 @@ export default function Home() {
             
             {/* Posts */}
             {(() => {
-              const posts = nodes.filter((n) => n.type === 'post');
+              const posts = nodes.filter((n) => n.type === 'post' && n.slug !== 'about');
               const pinnedPosts = posts.filter((post: any) => post.pinned);
               const unpinnedPosts = posts.filter((post: any) => !post.pinned);
               return [...pinnedPosts, ...unpinnedPosts].map((post: any) => (
@@ -341,7 +351,7 @@ export default function Home() {
                       )}
                       <div>Erstellt: {format(new Date(post.createdAt), 'd. MMMM yyyy HH:mm')}</div>
                     </div>
-                    <p className="text-gray-700 mb-3 sm:mb-4 text-sm sm:text-base">{post.summary}</p>
+                    <p className="text-gray-700 mb-3 sm:mb-4 text-sm sm:text-base">{stripFrontmatter(post.summary)}</p>
                     <div className="flex flex-wrap gap-1 sm:gap-2">
                       {post.tags.map((tag: string) => (
                         <span