feat(builder): CodeMirror 6 code editor for HTML/CSS/JS with lazy loading

Replaces the plain single-line input (HtmlBlock's `code` prop, via
GenericPropsEditor) and the plain <textarea> (HeadCodeModal's site-wide
head code) with a proper syntax-highlighted, tab-completing code editor.

- New src/ui/CodeEditor.tsx: reusable CodeMirror 6 editor (state/view/
  commands/autocomplete/language/lang-html/lang-css/lang-javascript/
  theme-one-dark). All @codemirror/* packages are pulled in via a single
  dynamic import() inside the component so they land in separate lazy
  chunks instead of the main bundle -- confirmed via `npm run build`:
  main editor.js grew by only ~5.5KB (657KB -> 663KB raw) while ~570KB of
  CodeMirror source split into index*.js chunks that only load when a
  code editor modal is actually opened. While that import is in flight,
  or if it ever fails, the component renders a plain <textarea> so typing
  never breaks.
- GenericPropsEditor.tsx: special-cases the `code` prop (used only by
  HtmlBlock today) into an "Edit HTML" button that opens the CodeEditor
  in a modal (language="html"), instead of rendering it as a single-line
  text input alongside the component's other string props.
- HeadCodeModal.tsx: swaps its <textarea> for CodeEditor (language="html"
  -- head code is HTML with embedded <script>/<style>), keeping the
  existing SiteDesignContext.updateDesign({ headCode }) wiring.

GuidedStyles.tsx untouched: HtmlBlock's displayName ("HTML") already
matches GuidedStyles' `isUtility` regex and routes to GenericPropsEditor,
so no dispatcher change was needed.

HtmlBlock.tsx untouched: purifyHtml/toHtml sanitization is unchanged, as
specified. Skipped the optional AnimationControl/VisibilityControl
addition -- HtmlBlock has no dedicated StylePanel (it shares
GenericPropsEditor with Divider/Spacer/every unmatched type), and doing
it well would mean adding real Animation/Visibility widgets to that
shared editor for every consumer, which is a bigger change than "trivial"
for this package's scope.

Tests: CodeEditor.test.tsx and HeadCodeModal.test.tsx both exploit the
fact that dynamic import() always resolves on a later microtask, so
asserting against the DOM immediately after the initial synchronous
render deterministically exercises the <textarea> fallback path (value
display, onChange wiring, language prop plumbing) without needing to
mock @codemirror/*.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

craft/src/ui/CodeEditor.test.tsx

@@ -0,0 +1,90 @@
+import { describe, test, expect, vi, afterEach } from 'vitest';
+import React from 'react';
+import { createRoot, Root } from 'react-dom/client';
+import { act } from 'react-dom/test-utils';
+import { CodeEditor } from './CodeEditor';
+
+/* ---------- DOM test harness (no @testing-library/react in this repo; see
+   src/ui/AssetPicker.test.tsx / src/ui/Modal.test.tsx for the same
+   react-dom/client + react-dom/test-utils `act` pattern). ----------
+
+   CodeMirror is loaded via dynamic import() (see CodeEditor.tsx), which is
+   always async -- even for an already-resolved/cached module, `import()`
+   only settles on a later microtask. That means immediately after the
+   initial synchronous `act(() => root.render(...))` below, the component is
+   still in its 'loading' state and renders the <textarea> fallback. These
+   tests deliberately assert against that first-tick DOM (never awaiting
+   the CodeMirror promise), so they exercise exactly the fallback path a
+   real headless/offline environment would fall back to, deterministically
+   and without needing to mock @codemirror/*. */
+
+let container: HTMLDivElement;
+let root: Root;
+
+function render(ui: React.ReactElement) {
+  container = document.createElement('div');
+  document.body.appendChild(container);
+  act(() => {
+    root = createRoot(container);
+    root.render(ui);
+  });
+}
+
+afterEach(() => {
+  if (container) {
+    act(() => { root.unmount(); });
+    container.remove();
+  }
+});
+
+function fallbackTextarea(): HTMLTextAreaElement {
+  const el = container.querySelector<HTMLTextAreaElement>('[data-testid="code-editor-fallback"]');
+  if (!el) throw new Error('fallback textarea not found');
+  return el;
+}
+
+describe('CodeEditor', () => {
+  test('shows the textarea fallback immediately (CodeMirror loads async)', () => {
+    render(<CodeEditor value="<p>hi</p>" onChange={vi.fn()} />);
+    const textarea = fallbackTextarea();
+    expect(textarea.value).toBe('<p>hi</p>');
+  });
+
+  test('the CodeMirror mount root is hidden while the fallback is showing', () => {
+    render(<CodeEditor value="" onChange={vi.fn()} />);
+    const cmRoot = container.querySelector<HTMLDivElement>('[data-testid="code-editor-cm-root"]');
+    expect(cmRoot?.style.display).toBe('none');
+  });
+
+  test('onChange fires with the new value when typing in the fallback textarea', () => {
+    const onChange = vi.fn();
+    render(<CodeEditor value="<p>hi</p>" onChange={onChange} />);
+    const textarea = fallbackTextarea();
+    act(() => {
+      const setter = Object.getOwnPropertyDescriptor(window.HTMLTextAreaElement.prototype, 'value')!.set!;
+      setter.call(textarea, '<p>updated</p>');
+      textarea.dispatchEvent(new Event('input', { bubbles: true }));
+    });
+    expect(onChange).toHaveBeenCalledWith('<p>updated</p>');
+  });
+
+  test('accepts a language prop without throwing (html/css/javascript/auto)', () => {
+    for (const language of ['html', 'css', 'javascript', 'auto'] as const) {
+      expect(() => render(<CodeEditor value="" onChange={vi.fn()} language={language} />)).not.toThrow();
+      const textarea = fallbackTextarea();
+      expect(textarea.dataset.language).toBe(language);
+      act(() => { root.unmount(); });
+      container.remove();
+    }
+  });
+
+  test('defaults to html language when none is passed', () => {
+    render(<CodeEditor value="" onChange={vi.fn()} />);
+    expect(fallbackTextarea().dataset.language).toBe('html');
+  });
+
+  test('respects a custom height', () => {
+    render(<CodeEditor value="" onChange={vi.fn()} height={480} />);
+    expect((container.firstElementChild as HTMLElement).style.height).toBe('480px');
+  });
+});