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/ui/CodeEditor/CodeEditor.module.css

@@ -0,0 +1,18 @@
+.wrapper {
+  width: 100%;
+  border: var(--border-width) solid var(--color-border);
+  border-radius: var(--radius-md);
+  overflow: hidden;
+  resize: vertical;
+  min-height: 80px;
+  transition: border-color var(--transition), box-shadow var(--transition);
+}
+
+.focused {
+  border-color: var(--color-primary);
+  box-shadow: 0 0 0 3px color-mix(in srgb, var(--color-primary) 20%, transparent);
+}
+
+.hasError {
+  border-color: var(--color-danger);
+}