feat: locale routing (en flat, zh subfolder), language switcher, dark default

- Derive lang from folder (posts/zh/* -> zh, else en); remove lang: frontmatter
- Change post route to [...slug] to capture nested zh paths
- Add nav language switcher (EN <-> 中文) + /zh/ landing page
- Default theme to dark (light opt-in via toggle)
- Translate hello-world-zh placeholder into English intro post
- Update case study: docker-compose sample + real install gotchas (commit history)
- Add docs/project-state.md (Tier 1/2/3 backlog)

src/pages/index.astro

@@ -2,9 +2,10 @@
 import BaseLayout from '../layouts/BaseLayout.astro';
 import PostList from '../components/PostList.astro';
 import { getCollection } from 'astro:content';
+import { isEn } from '../lib/lang';
 import { SITE } from '../config';
 
-const posts = (await getCollection('posts', ({ data }) => !data.draft && data.lang === 'en'))
+const posts = (await getCollection('posts', isEn))
   .sort((a, b) => b.data.pubDate.valueOf() - a.data.pubDate.valueOf());
 ---