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/snippet/SnippetsPage.tsx

@@ -1,7 +1,7 @@
 import { useEffect, useRef, useState } from 'react';
 import { useNavigate } from 'react-router-dom';
 import { useTranslation } from 'react-i18next';
-import { Code, Pencil, Trash2, Plus, Upload } from 'lucide-react';
+import { Code, Pencil, Trash2, Plus, Download } from 'lucide-react';
 import { parse as yamlParse } from 'yaml';
 import { snippets } from '../../api';
 import type { Snippet } from '../../api';
@@ -134,7 +134,7 @@ export function SnippetsPage() {
             onChange={handleImport}
           />
           <Button variant="secondary" size="sm" onClick={() => fileInputRef.current?.click()}>
-            <Upload size={14} />
+            <Download size={14} />
             {t('snippets.action_import')}
           </Button>
         </div>