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/panels/topbar/HeadCodeModal.test.tsx

@@ -0,0 +1,85 @@
+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 { HeadCodeModal } from './HeadCodeModal';
+import { SiteDesignProvider, useSiteDesign } from '../../state/SiteDesignContext';
+
+/* ---------- DOM test harness -- same react-dom/client + `act` pattern used
+   throughout src/ui/*.test.tsx (no @testing-library/react in this repo). ---------- */
+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();
+  }
+  document.body.style.overflow = '';
+});
+
+// Exposes the current headCode so assertions can read it back after a
+// simulated edit -- CodeEditor writes through `updateDesign`, this just
+// surfaces the result.
+function Harness({ onReady }: { onReady: (headCode: string) => void }) {
+  const { design } = useSiteDesign();
+  onReady(design.headCode);
+  return null;
+}
+
+describe('HeadCodeModal', () => {
+  // HeadCodeModal portals its content to document.body (see the comment in
+  // HeadCodeModal.tsx), so the rendered DOM lives outside `container` --
+  // query document.body instead.
+
+  test('renders the CodeEditor (fallback textarea path) seeded with the current headCode', () => {
+    render(
+      <SiteDesignProvider>
+        <HeadCodeModal open onClose={vi.fn()} />
+      </SiteDesignProvider>,
+    );
+    // CodeMirror loads via async dynamic import (see CodeEditor.test.tsx);
+    // synchronously after mount the fallback textarea is what's live.
+    const textarea = document.body.querySelector<HTMLTextAreaElement>('[data-testid="code-editor-fallback"]');
+    expect(textarea).not.toBeNull();
+    expect(textarea!.value).toBe('');
+    expect(textarea!.dataset.language).toBe('html');
+  });
+
+  test('typing in the editor writes through to SiteDesignContext.headCode', () => {
+    let latestHeadCode = '';
+    render(
+      <SiteDesignProvider>
+        <HeadCodeModal open onClose={vi.fn()} />
+        <Harness onReady={(v) => { latestHeadCode = v; }} />
+      </SiteDesignProvider>,
+    );
+
+    const textarea = document.body.querySelector<HTMLTextAreaElement>('[data-testid="code-editor-fallback"]')!;
+    act(() => {
+      const setter = Object.getOwnPropertyDescriptor(window.HTMLTextAreaElement.prototype, 'value')!.set!;
+      setter.call(textarea, '<meta name="x" content="y">');
+      textarea.dispatchEvent(new Event('input', { bubbles: true }));
+    });
+
+    expect(latestHeadCode).toBe('<meta name="x" content="y">');
+  });
+
+  test('does not render when closed', () => {
+    render(
+      <SiteDesignProvider>
+        <HeadCodeModal open={false} onClose={vi.fn()} />
+      </SiteDesignProvider>,
+    );
+    expect(document.body.querySelector('[data-testid="code-editor-fallback"]')).toBeNull();
+  });
+});