fix(html): resolve validation errors, implement dynamic MDX heading shifting, and improve accessibility

lib/blog.ts

@@ -221,10 +221,10 @@ export function getTextContent(node: any): string {
 }
 
 export function getHeadings(content: string): { id: string; text: string; level: number }[] {
-  const headingLines = content.split('\n').filter((line) => line.match(/^#{2,3}\s/));
+  const headingLines = content.split('\n').filter((line) => line.match(/^#{1,3}\s/));
 
   return headingLines.map((line) => {
-    const level = line.match(/^#+/)?.[0].length || 0;
+    const level = (line.match(/^#+/)?.[0].length || 0) + 1; // Shift H1->H2, H2->H3, H3->H4
     const rawText = line.replace(/^#+\s/, '').trim();
     const cleanText = rawText.replace(/[*_`]/g, '');
     const id = generateHeadingId(cleanText);