feat(i18n): default the cn build to Chinese

The .cn site exists for Chinese readers, so the unprefixed routes should
serve Chinese there. Rspress omits the prefix for the default locale, so
this also moves the routes:

              .com                      .cn
  English     /                         /en-gb/
  Chinese     /zh-hans/                 /

- Make the default locale a per-target value rather than a constant, and
  move the localePrefix helper inside createConfig so a prefix can no
  longer be hardcoded elsewhere.
- Build the sitemap priority map from localePrefix. The keys are full
  route paths, so the previous hardcoded "/zh-hans/..." entries would
  have silently stopped matching on .cn.
- Widen the nav Home activeMatch to cover either prefixed home, since the
  two targets put a different locale behind the prefix.

Note: README.md also carries the licence wording changed in the next
commit — a single file's hunks cannot be split non-interactively.

README.md

@@ -42,14 +42,28 @@ pnpm format     # Prettier — format all files
 
 The site is published on two domains, and the two builds are not
 interchangeable. `pnpm build:cn` runs `rspress build --config
-rspress.cn.config.ts`, which differs from the default in three ways:
+rspress.cn.config.ts`, which differs from the default in four ways:
 
 |                                 | `build:com`       | `build:cn`     |
 | ------------------------------- | ----------------- | -------------- |
+| Default language (unprefixed)   | `en-gb`           | `zh-hans`      |
 | Canonical, `hreflang`, `og:url` | `onixbyte.com`    | `onixbyte.cn`  |
 | `sitemap.xml`, `robots.txt`     | `onixbyte.com`    | `onixbyte.cn`  |
 | Social links                    | GitHub, Douyin, X | GitHub, Douyin |
 
+**The default language differs per target**, so the routes do too. Rspress
+serves the default locale without a prefix:
+
+|                 | `.com`                          | `.cn`                         |
+| --------------- | ------------------------------- | ----------------------------- |
+| English home    | `/`                             | `/en-gb/`                     |
+| Chinese home    | `/zh-hans/`                     | `/`                           |
+| English article | `/blogs/git-cheatsheet`         | `/en-gb/blogs/git-cheatsheet` |
+| Chinese article | `/zh-hans/blogs/git-cheatsheet` | `/blogs/git-cheatsheet`       |
+
+Content paths under `docs/` are unaffected — only the prefixes change, and
+Rspress rewrites internal links per locale automatically.
+
 X is omitted from the `.cn` build because it is not reachable from mainland
 China. Which links count as domestic is a single list in `rspress.config.ts`
 (`SOCIAL_LINKS`) — move an entry between the two arrays to change a build.
@@ -73,7 +87,7 @@ second build overwrites the first. To preview the `.cn` variant in dev, run
 docs/
 ├── public/              # Static assets served from the site root
 │   └── onixbyte-*.svg
-├── en-gb/               # English content (default locale, no URL prefix)
+├── en-gb/               # English content
 │   ├── _nav.json        # Top navigation; labels are i18n keys
 │   ├── index.md         # Marketing homepage
 │   ├── blogs/           # Technical articles ("Insights")
@@ -89,6 +103,7 @@ theme/                   # Custom Rspress theme
 ├── index.css            # Brand colours and section styling
 ├── lib/
 │   ├── site.ts          # Shared site constants
+│   ├── i18n.ts          # Types i18n.json for useI18n()
 │   └── social-icons.ts  # Inline SVG for social links with no preset icon
 └── components/
     ├── Author.tsx       # Byline, rendered from `author` frontmatter
@@ -146,6 +161,16 @@ Redirects for the retired `/projects/` paths live in `vercel.json`. The `.cn`
 host needs the equivalent rule in its own web server config, which is not
 tracked in this repository.
 
+## Contributing
+
+This repository is **not open for external contributions** — pull requests are
+not accepted. Issues reporting broken links, factual errors, typos or
+reproducible bugs are welcome. See [CONTRIBUTING.md](./CONTRIBUTING.md).
+
 ## Licence
 
-MIT
+All rights reserved. Nothing here is licensed for reuse: no permission is
+granted to copy, modify or distribute any part of this repository. Reading it
+and linking to it are fine. See [LICENCE](./LICENCE) for the full notice, and
+[THIRD_PARTY_NOTICES.md](./THIRD_PARTY_NOTICES.md) for the dependencies, which
+keep their own licences.