fix: sync AVAILABLE_LANGUAGES with shipped locales (add zh-CN; drop duplicate ja)

[everysingletear]

What

The language selector reads AVAILABLE_LANGUAGES in
src/contexts/LocalizationContext.jsx, which is out of sync with the locale
files in public/locales/:

• Adds Simplified Chinese (zh-CN). It ships a complete translation (key
  structure identical to en) but was unreachable in the selector — zh-CN
  landed in feat: Add Simplified Chinese (zh-CN) localization新增简体中文(zh-CN)语言汉化包 #132 without a selector entry.
• Removes the duplicate Japanese (ja) entry (two merges of the "add
  Japanese" work each appended one).

Note

Arabic (ar) was in the original version of this PR but has been dropped per
maintainer feedback — surfacing it activates the existing RTL path
(RTL_LANGUAGES already includes ar), and RTL needs a proper pass first. It
can come back as its own PR once RTL is verified. This PR is now purely the
zh-CN fix + ja de-dup, with no RTL implications.

How to verify

• The selector lists Chinese (Simplified) and loads its dictionary with no
  missing keys.
• Japanese appears once, not twice.

[meauxt]

I think we can remove arabic as the RTL seems need more work

[everysingletear]

Good call — dropped Arabic and force-pushed. RTL deserves a proper visual pass before we surface ar, so I'll bring it back as its own PR once that's verified. This PR is now just the zh-CN fix + ja de-dup.