feat(ui): add CodeBlock and CodeEditor components with Monaco and hljs

- add CodeBlock for read-only syntax-highlighted display (hljs, js/json)
- add CodeEditor wrapping Monaco editor with theme sync, focus state, and error state
- replace code textareas in snippet, credential, and step pages with CodeEditor
- replace code <pre> blocks in snippet and credential detail pages with CodeBlock
- make form cards full-width on pages with code editors
- add resize:vertical support to CodeEditor wrapper with automaticLayout

client/src/pages/scenario/ScenarioDetailPage.tsx

@@ -1,7 +1,7 @@
 import { useEffect, useState } from 'react';
 import { useNavigate, useParams } from 'react-router-dom';
 import { useTranslation } from 'react-i18next';
-import { Play, Pencil, Plus, History, Trash2, Download } from 'lucide-react';
+import { Play, Pencil, Plus, History, Trash2, Upload } from 'lucide-react';
 import { stringify as yamlStringify } from 'yaml';
 import { scenarios, steps, scenarioCredentials, credentials as credentialsApi } from '../../api';
 import type { Scenario, ScenarioStep, ScenarioCredential, Credential } from '../../api';
@@ -227,7 +227,7 @@ export function ScenarioDetailPage() {
               {t('scenarios.action_runs')}
             </Button>
             <Button variant="secondary" size="sm" onClick={handleExport}>
-              <Download size={14} />
+              <Upload size={14} />
               {t('scenarios.action_export')}
             </Button>
             <Button variant="secondary" size="sm" onClick={() => navigate(`/scenarios/${id}/edit`)}>