fix(i18n,404,nginx): broken-windows batch B2 from the external review

All items measured before/after with curl, on the origin and through Cloudflare:

- /posts/zh/ answered 403 (directory with no index) -> nginx 301 to the new archive
- /zh/posts/ 404'd -> new src/pages/zh/posts/index.astro (build 163 -> 165 pages)
- ZH nav pointed "关于" at /zh/ and "文章" at /zh/ -> /zh/about/ and /zh/posts/
- og:locale was "en" on every Chinese page -> zh_CN, plus og:locale:alternate on
  both languages (EN pages now advertise zh_CN as well)
- hreflang "zh" -> "zh-Hans"; <html lang> on ZH pages -> zh-Hans
- no 404 page existed -> src/pages/404.astro, served by nginx with a 404 status so
  crawlers skip it, offering search + archive + the 3 newest posts
- JSON-LD Person gained worksFor (Hoelee Enterprise); image still omitted until the
  author photo exists rather than pointing at a placeholder

nginx: config moved from the stock image file to a host bind mount
(/mnt/user/docker/nginx-hoelee-blog/conf.d/default.conf, Portainer stack 247 on
endpoint 3), validated with nginx -t in a throwaway container before the redeploy.
It also sets /_astro/* to immutable 1-year (hashed names) and HTML to
must-revalidate; unhashed /og/*.png and /banners/*.png keep the default TTL on
purpose. Note: the public HSTS and 16-day asset TTL come from Cloudflare zone
settings, not from this nginx.

Docs: ops-runbook.md (the nginx layer + why each rule), project-state.md (B2 entry
and the two open Cloudflare items my token cannot write).

src/styles/global.css

@@ -329,6 +329,26 @@ pre code {
 }
 .related-card time { font-size: 0.78rem; margin-top: auto; padding-top: 0.4rem; }
 
+/* ---------- 404 ---------- */
+.notfound-actions {
+  display: flex;
+  flex-wrap: wrap;
+  align-items: center;
+  gap: 0.75rem;
+  margin: 1.5rem 0 2rem;
+}
+.notfound-btn {
+  font: inherit;
+  color: var(--ink);
+  background: var(--surface);
+  border: 1px solid var(--border);
+  border-radius: var(--radius);
+  padding: 0.45rem 0.9rem;
+  cursor: pointer;
+}
+.notfound-btn:hover { border-color: var(--brand); color: var(--brand); }
+.notfound-row .body { padding-block: 0.6rem; }
+
 /* ---------- Post list ---------- */
 .post-list { list-style: none; padding: 0; margin: 0; }
 .post-list li {