Dead link in How I Back Up: onyxboox.com is gone

## Summary

`src/data/blog/how-i-back-up.md:85` points the Tab Ultra C reference at a domain that no longer answers:

```
[boox]: https://onyxboox.com/boox_tabultrac
```

The whole domain is unreachable, not just that path — `https://onyxboox.com/` times out at 25s with a browser user agent, so it isn't bot protection rejecting CI.

Onyx appears to have moved to `boox.com`. `https://shop.boox.com/products/tabultrac` serves 200 and looks like the equivalent product page, though I haven't checked it's the same model variant the post means. Worth a look before swapping it in — `https://www.boox.com/` itself fails a TLS handshake ("dh key too small"), which suggests the estate is mid-migration and the shop subdomain may not be the destination they settle on.

## Context

Surfaced by the weekly link scan added in #483, which is the first check to reach this link — it sat under a `lychee.toml` `exclude_path` entry until now. It's the only genuinely broken link on the published site; everything else that scan reported was either bot protection or checker tuning.