feat(client): add i18n with i18next and react-i18next

- install i18next and react-i18next
- create src/i18n/index.ts bootstrapping i18next with initReactI18next
- add src/i18n/locales/en.json with keys for nav, all pages, and ThemeSwitcher
- replace hardcoded English strings in App, all pages, and ThemeSwitcher with t()
- move static column definitions inside components where useTranslation is available

client/src/ui/ThemeSwitcher/ThemeSwitcher.tsx

@@ -1,16 +1,18 @@
 import { Sun, Moon } from 'lucide-react';
+import { useTranslation } from 'react-i18next';
 import { useTheme } from '../../hooks/useTheme';
 import styles from './ThemeSwitcher.module.css';
 
 export function ThemeSwitcher() {
   const { theme, toggleTheme } = useTheme();
+  const { t } = useTranslation();
 
   return (
     <button
       className={styles.btn}
       onClick={toggleTheme}
-      aria-label={theme === 'dark' ? 'Switch to light theme' : 'Switch to dark theme'}
-      title={theme === 'dark' ? 'Light mode' : 'Dark mode'}
+      aria-label={theme === 'dark' ? t('theme.switch_to_light') : t('theme.switch_to_dark')}
+      title={theme === 'dark' ? t('theme.light_mode') : t('theme.dark_mode')}
     >
       {theme === 'dark' ? <Sun size={16} /> : <Moon size={16} />}
     </button>