feat(i18n): add localized desktop UI

Refs #17

src/i18n/locales.test.ts

@@ -2,14 +2,16 @@ import { describe, expect, it } from 'vitest';
 import {
   APP_LOCALES,
   DEFAULT_LOCALE,
+  isAppLocalePreference,
   isAppLocale,
   localeDirection,
+  localePluralVariant,
   resolveAppLocale,
 } from './locales';
 
 describe('app locale metadata', () => {
   it('keeps the planned locale set stable', () => {
-    expect(APP_LOCALES).toEqual(['en', 'zh-CN', 'he', 'fa', 'uk']);
+    expect(APP_LOCALES).toEqual(['en', 'zh-CN', 'he', 'fa', 'uk', 'ru']);
     expect(DEFAULT_LOCALE).toBe('en');
   });
 
@@ -19,6 +21,7 @@ describe('app locale metadata', () => {
     expect(resolveAppLocale('iw-IL')).toBe('he');
     expect(resolveAppLocale('fa-IR')).toBe('fa');
     expect(resolveAppLocale('uk-UA')).toBe('uk');
+    expect(resolveAppLocale('ru-RU')).toBe('ru');
     expect(resolveAppLocale('de-DE')).toBe('en');
   });
 
@@ -28,12 +31,26 @@ describe('app locale metadata', () => {
     expect(localeDirection('he')).toBe('rtl');
     expect(localeDirection('fa')).toBe('rtl');
     expect(localeDirection('uk')).toBe('ltr');
+    expect(localeDirection('ru')).toBe('ltr');
+  });
+
+  it('selects locale-aware plural categories', () => {
+    expect(localePluralVariant('en', 1)).toBe('one');
+    expect(localePluralVariant('en', 2)).toBe('many');
+    expect(localePluralVariant('ru-RU', 1)).toBe('one');
+    expect(localePluralVariant('ru-RU', 2)).toBe('few');
+    expect(localePluralVariant('ru-RU', 5)).toBe('many');
+    expect(localePluralVariant('uk-UA', 2)).toBe('few');
   });
 
   it('guards locale values before they reach i18next', () => {
     expect(isAppLocale('en')).toBe(true);
     expect(isAppLocale('fa')).toBe(true);
+    expect(isAppLocale('ru')).toBe(true);
     expect(isAppLocale('de')).toBe(false);
     expect(isAppLocale(null)).toBe(false);
+    expect(isAppLocalePreference('system')).toBe(true);
+    expect(isAppLocalePreference('fa')).toBe(true);
+    expect(isAppLocalePreference('de')).toBe(false);
   });
 });