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/right/styles/GenericPropsEditor.tsx

@@ -1,4 +1,4 @@
-import React from 'react';
+import React, { useState } from 'react';
 import {
   TEXT_COLORS,
   BG_COLORS,
@@ -17,12 +17,85 @@ import {
   useNodeProp,
 } from './shared';
 import { ArrayItemFieldsEditor } from './ArrayItemFields';
+import { Modal } from '../../../ui/Modal';
+import { CodeEditor } from '../../../ui/CodeEditor';
+
+/* ---------- "Edit HTML" modal for the HtmlBlock `code` prop ----------
+   `code` is raw HTML (potentially many lines, embedded <style>/<script>),
+   so it gets a dedicated syntax-highlighted CodeEditor in a modal instead
+   of falling into the generic single-line/textarea string-prop rendering
+   below (see GenericPropsEditor's SKIP of the `code` key). */
+const HtmlCodeField: React.FC<{ value: string; onChange: (v: string) => void }> = ({ value, onChange }) => {
+  const [open, setOpen] = useState(false);
+  return (
+    <CollapsibleSection title="HTML Code">
+      <div style={sectionGap}>
+        <button
+          onClick={() => setOpen(true)}
+          style={{
+            width: '100%', padding: '8px 12px', fontSize: 12, fontWeight: 600,
+            display: 'flex', alignItems: 'center', justifyContent: 'center', gap: 6,
+            background: '#27272a', color: '#e4e4e7', border: '1px solid #3f3f46',
+            borderRadius: 6, cursor: 'pointer',
+          }}
+        >
+          <i className="fa fa-code" /> Edit HTML
+        </button>
+      </div>
+      <Modal open={open} onClose={() => setOpen(false)} width="min(720px, 90vw)">
+        <div
+          style={{
+            background: 'var(--color-bg-surface)',
+            border: '1px solid var(--color-border)',
+            borderRadius: 12,
+            boxShadow: '0 20px 60px rgba(0,0,0,0.5)',
+            display: 'flex',
+            flexDirection: 'column',
+            overflow: 'hidden',
+          }}
+          onClick={(e) => e.stopPropagation()}
+        >
+          <div style={{
+            display: 'flex', alignItems: 'center', justifyContent: 'space-between',
+            padding: '14px 16px', borderBottom: '1px solid var(--color-border)',
+          }}>
+            <div style={{ fontSize: 14, fontWeight: 600, color: 'var(--color-text)' }}>Edit HTML</div>
+            <button
+              onClick={() => setOpen(false)}
+              style={{
+                width: 28, height: 28, display: 'inline-flex', alignItems: 'center', justifyContent: 'center',
+                background: 'none', border: '1px solid var(--color-border)', borderRadius: 6,
+                color: 'var(--color-text-muted)', cursor: 'pointer', fontSize: 13,
+              }}
+            >
+              <i className="fa fa-times" />
+            </button>
+          </div>
+          <div style={{ padding: 16 }}>
+            <CodeEditor value={value} onChange={onChange} language="html" height={420} />
+          </div>
+          <div style={{ padding: '10px 16px', borderTop: '1px solid var(--color-border)', display: 'flex', justifyContent: 'flex-end' }}>
+            <button
+              onClick={() => setOpen(false)}
+              style={{
+                padding: '7px 20px', fontSize: 13, fontWeight: 600,
+                background: 'var(--color-accent)', color: '#fff', border: 'none', borderRadius: 6, cursor: 'pointer',
+              }}
+            >
+              Done
+            </button>
+          </div>
+        </div>
+      </Modal>
+    </CollapsibleSection>
+  );
+};
 
 /* ---------- SMART GENERIC PROPS EDITOR (Fallback) ---------- */
 export const GenericPropsEditor: React.FC<{ selectedId: string; nodeProps: Record<string, any>; typeName: string }> = ({
   selectedId, nodeProps, typeName,
 }) => {
-  const SKIP_PROPS = new Set(['style', 'children', 'cssId', 'cssClass']);
+  const SKIP_PROPS = new Set(['style', 'children', 'cssId', 'cssClass', 'code']);
 
   const { setProp: setPropValue, setPropStyle: setStyleValue } = useNodeProp(selectedId);
 
@@ -35,9 +108,15 @@ export const GenericPropsEditor: React.FC<{ selectedId: string; nodeProps: Recor
   const arrayProps = allProps.filter(([_, val]) => Array.isArray(val));
 
   const style = nodeProps.style || {};
+  const hasCodeProp = typeof nodeProps.code === 'string';
 
   return (
     <>
+      {/* Raw HTML (HtmlBlock's `code` prop) -- dedicated CodeEditor modal */}
+      {hasCodeProp && (
+        <HtmlCodeField value={nodeProps.code} onChange={(v) => setPropValue('code', v)} />
+      )}
+
       {/* String props */}
       {stringProps.length > 0 && (
         <CollapsibleSection title="Properties">