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/i18n/locales/en.json

@@ -0,0 +1,45 @@
+{
+  "nav": {
+    "environments": "Environments",
+    "keys": "Keys",
+    "sessions": "Sessions",
+    "scenarios": "Scenarios"
+  },
+  "environments": {
+    "title": "Environments",
+    "col_id": "ID",
+    "col_name": "Name",
+    "col_urls": "URLs",
+    "col_updated": "Updated",
+    "empty": "No environments yet."
+  },
+  "keys": {
+    "title": "Keys",
+    "col_name": "Key name",
+    "empty": "No key files found."
+  },
+  "sessions": {
+    "title": "Sessions",
+    "col_id": "ID",
+    "col_name": "Name",
+    "col_status": "Status",
+    "col_lastUsed": "Last Used",
+    "empty": "No sessions yet.",
+    "action_delete": "Delete"
+  },
+  "scenarios": {
+    "title": "Scenarios",
+    "col_id": "ID",
+    "col_name": "Name",
+    "col_updated": "Updated",
+    "empty": "No scenarios yet.",
+    "action_run": "Run",
+    "action_delete": "Delete"
+  },
+  "theme": {
+    "switch_to_light": "Switch to light theme",
+    "switch_to_dark": "Switch to dark theme",
+    "light_mode": "Light mode",
+    "dark_mode": "Dark mode"
+  }
+}