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/credential/CredentialDetailPage.tsx

@@ -1,7 +1,8 @@
 import { useEffect, useState } from 'react';
 import { useNavigate, useParams } from 'react-router-dom';
 import { useTranslation } from 'react-i18next';
-import { Download, Pencil, Trash2 } from 'lucide-react';
+import { Upload, Pencil, Trash2 } from 'lucide-react';
+import { CodeBlock } from '../../ui';
 import { stringify as yamlStringify } from 'yaml';
 import { credentials } from '../../api';
 import type { Credential } from '../../api';
@@ -55,7 +56,7 @@ export function CredentialDetailPage() {
         {credential && (
           <div className={styles.toolbarActions}>
             <Button variant="secondary" size="sm" onClick={handleExport}>
-              <Download size={14} />
+              <Upload size={14} />
               {t('credentials.action_export')}
             </Button>
             <Button
@@ -116,15 +117,16 @@ export function CredentialDetailPage() {
             <>
               <h2 className={styles.sectionHeading}>{t('credentials.section_data')}</h2>
               <Card>
-                <pre className={styles.codeBlock}>
-                  {(() => {
+                <CodeBlock
+                  language="json"
+                  code={(() => {
                     try {
                       return JSON.stringify(JSON.parse(credential.data), null, 2);
                     } catch {
                       return credential.data;
                     }
                   })()}
-                </pre>
+                />
               </Card>
             </>
           )}