Review follow-ups on #312:
- `crates/ui-egui/src/theme.rs`'s `japanese_font_tests` still called `install` with two
arguments, so `cargo test` did not compile. Every call site now passes a language.
- The call sites passed the stored `general.language` preference. With the default `system`
that is `""`, which handed the CJK choice back to the environment locale — one macOS and
Windows usually do not set. They now pass `crate::i18n::language(app)`, the resolved
language, so Match System draws Chinese on a Chinese system.
- Changing Settings ▸ General ▸ Language only reached the fonts on the first frame.
`EffectcraftApp` now remembers the language its fonts were installed for and installs them
again when it changes, so the switch applies without a restart.
- Documented that `fonts::set_cjk_locale` is process-global: the Han preference therefore
applies to composition text as well as the interface chrome (docs/preferences.md and the
function's own comment).
Rebased onto main after #309 landed — `header.rs` gained an `install` call site in the
meantime, and the catalogs now resolve through `i18n::language`.
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