text, ui: follow a language change, and resolve the language before the fonts

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`.

crates/text/src/fonts.rs

@@ -576,7 +576,8 @@ static REQUESTED_CJK_LOCALE: RwLock<Option<String>> = RwLock::new(None);
 /// environment, so a Chinese interface is drawn with a Chinese face on a machine whose locale is
 /// Japanese (and the other way round). The interface sets this from its language setting; an empty
 /// tag, `c` or `posix` clears the request and returns to the environment. Process-global, like the
-/// environment it replaces.
+/// environment it replaces: every layout in the process follows it, so composition text is drawn
+/// with the same language's Han forms as the interface, not only the interface chrome.
 pub fn set_cjk_locale(tag: &str) {
     let tag = tag.trim().to_ascii_lowercase();
     let requested = if tag.is_empty() || tag == "c" || tag == "posix" { None } else { Some(tag) };