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/main.tsx

@@ -1,5 +1,6 @@
 import { StrictMode } from 'react'
 import { createRoot } from 'react-dom/client'
+import './i18n'
 import './ui/tokens.css'
 import App from './App'