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/lib/hljs.ts

@@ -0,0 +1,12 @@
+import hljs from 'highlight.js/lib/core';
+import javascript from 'highlight.js/lib/languages/javascript';
+import json from 'highlight.js/lib/languages/json';
+
+hljs.registerLanguage('javascript', javascript);
+hljs.registerLanguage('json', json);
+
+export default hljs;
+
+export function escapeHtml(str: string): string {
+  return str.replace(/&/g, '&amp;').replace(/</g, '&lt;').replace(/>/g, '&gt;');
+}