fix: prevent language switch state reset caused by dependency cycle

Fixed an issue where clicking the language switcher would cause a brief flash
and fail to persist the language change. The root cause was a dependency cycle
in useSettingsForm where readPersistedLanguage depended on i18n.language,
causing the initialization effect to re-run and reset state whenever the
language changed.

Changed the dependency from [i18n.language] to [i18n] since the i18n object
itself is stable and doesn't change when the language changes, while the
function can still access the current language value via closure.

src/hooks/useSettingsForm.ts

@@ -56,7 +56,7 @@ export function useSettingsForm(): UseSettingsFormResult {
       }
     }
     return normalizeLanguage(i18n.language);
-  }, [i18n.language]);
+  }, [i18n]);
 
   const syncLanguage = useCallback(
     (lang: Language) => {