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/package.json

@@ -11,9 +11,11 @@
     "build-storybook": "storybook build"
   },
   "dependencies": {
+    "i18next": "^26.0.4",
     "lucide-react": "^1.7.0",
     "react": "^19.1.0",
-    "react-dom": "^19.1.0"
+    "react-dom": "^19.1.0",
+    "react-i18next": "^17.0.2"
   },
   "devDependencies": {
     "@storybook/addon-a11y": "^10.3.5",