i18n: the browser build's Match System uses supported() too (#229)

`supported` was only called on desktop builds, so the wasm32 check
warned that it was never used. The browser build, which has no system
locale here, now resolves Match System through supported(None)
(English) instead of a literal.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/ui-egui/src/i18n.rs

@@ -22,8 +22,9 @@ fn system_language() -> &'static str {
         static LANGUAGE: std::sync::OnceLock<&'static str> = std::sync::OnceLock::new();
         LANGUAGE.get_or_init(|| supported(sys_locale::get_locale().as_deref()))
     }
+    // No system locale in the browser build: English.
     #[cfg(target_arch = "wasm32")]
-    "en"
+    supported(None)
 }
 
 /// The language EffectCraft shows for a BCP 47 locale (`ja-JP` → `ja`).