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)

README.md

@@ -26,18 +26,22 @@ Every decision below optimizes for **credibility + discoverability + speed of sh
 ```
 src/
   content/
-    posts/          # blog posts (en + zh)
+    posts/          # English posts (flat)
+      zh/           # Chinese posts (subfolder -> /posts/zh/<slug>/)
   pages/
     index.astro     # homepage
     posts/[...slug].astro   # post template
     about.astro
   layouts/
     BaseLayout.astro
+  lib/
+    lang.ts         # derives en/zh from folder path
 docs/
   content-guide.md  # what to write, what to avoid (READ before writing a post)
   design-guide.md   # what the design must have, what to avoid (READ before touching UI)
   seo-reference.md  # E-E-A-T / name identity, SEO + GEO checklist, syndication
   ops-runbook.md    # pipeline, publish steps, build pitfalls, health checks
+  project-state.md  # enhancement backlog (Tier 1/2/3) — work through one by one
 ```
 
 ## Knowledge guides (my rules of thumb)
@@ -48,10 +52,11 @@ docs/
 | [`docs/design-guide.md`](docs/design-guide.md) | Before changing theme, layout, typography, color, or SEO markup. |
 | [`docs/seo-reference.md`](docs/seo-reference.md) | Before adding posts/`<head>` markup or debugging search visibility. |
 | [`docs/ops-runbook.md`](docs/ops-runbook.md) | Before debugging CI/CD, deployment, or hosting. |
+| [`docs/project-state.md`](docs/project-state.md) | To see what's next — the prioritized enhancement backlog. |
 
 ## Write a post
 
-1. Create `src/content/posts/<slug>.md` with frontmatter (see content guide).
+1. Create `src/content/posts/<slug>.md` (English) or `src/content/posts/zh/<slug>.md` (Chinese) with frontmatter (see content guide — **no `lang:` field**, it's derived from the folder).
 2. Push to `main`.
 3. CI builds and deploys automatically.
 
@@ -62,5 +67,7 @@ docs/
 ## Conventions
 
 - **Repo publishing order:** push to git.hoelee.com (Gitea) first, then GitHub. (My standing convention.)
+- **Language:** English posts flat in `posts/`; Chinese posts in `posts/zh/`; no Malay.
+- **Theme:** defaults to dark mode (light is opt-in via the toggle).
 - **No overclaiming** — especially Web3 (learning projects, not production DeFi).
 - **Name identity is consistent everywhere:** "Lee Teong Hoe" / "Mr Hoelee" + same photo + same `sameAs` handles across blog, LinkedIn, GitHub, git.hoelee.com.