Categories page: descriptions, per-category terminal illustrations, full ZH version

- 7 categories now listed with name, blurb, post count and a
  per-category terminal-style SVG illustration (CategoryArt)
- empty categories show '0 posts · coming soon' (non-link, no soft-404)
- new zh pages: /zh/categories/ index + /zh/categories/[category]/ detail
- PostList is locale-aware (zh-CN dates, zh category links)
- zh nav '分类' now points to /zh/categories/
- category detail pages get art + description header
- language switch wired both ways (EN <-> ZH)

src/layouts/BaseLayout.astro

@@ -135,7 +135,7 @@ const nav = isZh
       <nav class="nav" aria-label="Main">
         <a class="brand" href="/">Mr<span class="dot"> Hoelee</span></a>
         <a href={isZh ? '/zh/' : '/posts/'}>{nav.posts}</a>
-        <a href={isZh ? '/zh/' : '/categories/'}>{nav.categories}</a>
+        <a href={isZh ? '/zh/categories/' : '/categories/'}>{nav.categories}</a>
         <a href={isZh ? '/zh/' : '/about/'}>{nav.about}</a>
         <a href="/rss.xml" title={nav.rss}>{nav.rss}</a>
         <button class="search-toggle" id="search-toggle" aria-label={isZh ? '搜索' : 'Search'}>⌕</button>