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/en.json

@@ -1056,8 +1056,8 @@
       "fieldExcerptEn": "Excerpt (English)",
       "fieldCategoryFa": "Category (Persian)",
       "fieldCategoryEn": "Category (English)",
-      "fieldContentFa": "Content (Persian, Markdown)",
-      "fieldContentEn": "Content (English, Markdown)",
+      "fieldContentFa": "Content (Persian)",
+      "fieldContentEn": "Content (English)",
       "fieldPublished": "Published",
       "commentsTitle": "Comment management",
       "noComments": "No comments found",
@@ -1219,7 +1219,10 @@
     "notFound": "Café not found",
     "exploreMore": "Explore more cafés",
     "reviewCount": "{count} reviews",
-    "cities": { "tehran": "Tehran", "karaj": "Karaj" },
+    "cities": {
+      "tehran": "Tehran",
+      "karaj": "Karaj"
+    },
     "sort": {
       "rating": "Top rated",
       "reviews": "Most reviews",