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/content.config.ts

@@ -18,7 +18,9 @@ const posts = defineCollection({
     ]),
     tags: z.array(z.string()).default([]),
     draft: z.boolean().default(false),
-    lang: z.enum(['en', 'zh']).default('en'),
+    // lang is NOT set in frontmatter anymore — it's derived from the folder:
+    //   posts/hello-world.md  -> en (flat)
+    //   posts/zh/hello-world.md -> zh (subfolder)
     ogImage: z.string().optional(),
   }),
 });