feat: add script to generate default blog cover image

- Introduced a new script `make-blog-cover.mjs` to create a default cover image for blog posts.
- The image is generated in PNG format with dimensions 1200x630, suitable for Open Graph.
- Utilizes the site's branding colors and logo from `public/nobat724.svg`.
- Includes custom font styling using the Vazirmatn font.
- The generated image is saved to `public/assets/images/blog-default-cover.png`.

lib/sanitize.js

@@ -12,6 +12,9 @@ export function sanitizeHtml(html) {
       "p", "br", "strong", "em", "b", "i", "u", "ul", "ol", "li", "a",
       "h2", "h3", "h4", "h5", "blockquote", "img", "span", "div",
       "table", "thead", "tbody", "tr", "td", "th",
+      // خروجی CKEditor پنل ادمین جدول و تصویر را داخل <figure> می‌گذارد؛ بدون
+      // این دو، wrapper حذف و figcaption (متن زیر تصویر) کاملاً گم می‌شد.
+      "figure", "figcaption",
     ],
     ALLOWED_ATTR: ["href", "target", "rel", "src", "alt", "title"],
     ALLOW_DATA_ATTR: false,