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/pages/ScenariosPage.tsx

@@ -1,10 +1,12 @@
 import { useCallback, useEffect, useState } from 'react';
+import { useTranslation } from 'react-i18next';
 import { scenarios } from '../api';
 import type { Scenario } from '../api';
 import { Button, Table, type TableColumn } from '../ui';
 import styles from './Page.module.css';
 
 export function ScenariosPage() {
+  const { t } = useTranslation();
   const [items, setItems] = useState<Scenario[]>([]);
   const [loading, setLoading] = useState(true);
   const [error, setError] = useState<string | null>(null);
@@ -29,11 +31,11 @@ export function ScenariosPage() {
   };
 
   const columns: TableColumn<Scenario>[] = [
-    { key: 'id',      header: 'ID',      render: (s) => s.id,   width: 60 },
-    { key: 'name',    header: 'Name',    render: (s) => s.name },
+    { key: 'id',      header: t('scenarios.col_id'),      render: (s) => s.id,   width: 60 },
+    { key: 'name',    header: t('scenarios.col_name'),    render: (s) => s.name },
     {
       key: 'updated',
-      header: 'Updated',
+      header: t('scenarios.col_updated'),
       width: 120,
       render: (s) => new Date(s.updatedAt).toLocaleDateString(),
     },
@@ -44,8 +46,8 @@ export function ScenariosPage() {
       align: 'right',
       render: (s) => (
         <span style={{ display: 'inline-flex', gap: 6 }}>
-          <Button size="sm" onClick={() => handleRun(s.id)}>Run</Button>
-          <Button variant="secondary" size="sm" onClick={() => handleDelete(s.id)}>Delete</Button>
+          <Button size="sm" onClick={() => handleRun(s.id)}>{t('scenarios.action_run')}</Button>
+          <Button variant="secondary" size="sm" onClick={() => handleDelete(s.id)}>{t('scenarios.action_delete')}</Button>
         </span>
       ),
     },
@@ -53,14 +55,14 @@ export function ScenariosPage() {
 
   return (
     <div>
-      <h1 className={styles.heading}>Scenarios</h1>
+      <h1 className={styles.heading}>{t('scenarios.title')}</h1>
       {error && <p className={styles.error}>{error}</p>}
       <Table
         columns={columns}
         data={items}
         rowKey={(s) => s.id}
         loading={loading}
-        emptyMessage="No scenarios yet."
+        emptyMessage={t('scenarios.empty')}
       />
     </div>
   );