feat(admin): rich text editor for blog content (TipTap)

Blog post bodies were plain <textarea>s labelled "Markdown". Replace with a
TipTap rich editor (bold/italic/strike, H1–H3, lists, blockquote, code, links,
undo/redo), RTL-aware, producing HTML.

- New RichTextEditor component (TipTap v2: react + starter-kit + pm + link +
  placeholder), immediatelyRender:false for Next SSR, self-contained content
  styling, external-value sync.
- Wired into the FA/EN content fields of the blog editor; labels no longer say
  "Markdown" (fa/en/ar).
- Website blog page now renders HTML when the body is HTML and falls back to
  MDXRemote for older Markdown posts (backward-compatible). Content is authored
  only by trusted SystemAdmins, so HTML is stored/rendered directly.

Admin build + website typecheck clean.

web/admin/messages/fa.json

@@ -1056,8 +1056,8 @@
       "fieldExcerptEn": "خلاصه انگلیسی",
       "fieldCategoryFa": "دسته‌بندی فارسی",
       "fieldCategoryEn": "دسته‌بندی انگلیسی",
-      "fieldContentFa": "محتوا فارسی (Markdown)",
-      "fieldContentEn": "محتوا انگلیسی (Markdown)",
+      "fieldContentFa": "محتوا (فارسی)",
+      "fieldContentEn": "محتوا (انگلیسی)",
       "fieldPublished": "وضعیت انتشار",
       "commentsTitle": "مدیریت نظرات",
       "noComments": "نظری یافت نشد",