fix(builder): sanitize non-string style values + allowlist element tags (XSS)

Adversarial re-review found the C1 fix incomplete plus an adjacent
same-class XSS, both reachable via the AI update_props path and
deserialized saved state:

- cssPropsToString only ran sanitizeCssValue on typeof-string values, so a
  non-string style value (array/object) with a valid key skipped
  sanitization entirely and was template-coerced raw into style="...",
  e.g. { color: ['red', '"><img src=x onerror=alert(1)>'] }. Now every
  non-number value is coerced with String() and sanitized; numbers stay
  raw. sanitizeBreakoutChars also now escapes < and > (previously only ;
  and ") as defense-in-depth, since values can reach it from non-string
  sources.

- props.tag (Container) and props.level (Heading) were interpolated raw
  into the tag position of exported HTML (`<${tag}`, `<${level}`) with no
  runtime validation, letting a malicious value break out of the tag
  entirely. Both are now allowlisted/clamped against their known-safe sets
  (div/section/article/header/footer/main; h1-h6), falling back to
  div/h2. Applied in Container's live render + toHtml, Heading's live
  render + toHtml, and the typeName==='div' fallback branch in
  html-export.ts's renderNode (hit for unresolved/legacy node types).

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

craft/src/components/basic/Heading.tsx

@@ -4,6 +4,16 @@ import { cssPropsToString } from '../../utils/style-helpers';
 
 type HeadingLevel = 'h1' | 'h2' | 'h3' | 'h4' | 'h5' | 'h6';
 
+// `level` is settable via the AI `update_props` path and from deserialized
+// saved state -- neither type-checked at runtime -- and is interpolated
+// directly into the tag position (`React.createElement(level, ...)` /
+// `<${tag}` in `toHtml`). A malicious value like `h2><img src=x
+// onerror=alert(1)` (or a non-h1-6 string) must never reach that position
+// unchecked. Anything not in this allowlist clamps to `'h2'`.
+const ALLOWED_HEADING_LEVELS = ['h1', 'h2', 'h3', 'h4', 'h5', 'h6'] as const;
+const sanitizeHeadingLevel = (level: unknown): HeadingLevel =>
+  (ALLOWED_HEADING_LEVELS as readonly unknown[]).includes(level) ? (level as HeadingLevel) : 'h2';
+
 interface HeadingProps {
   text?: string;
   level?: HeadingLevel;
@@ -30,6 +40,7 @@ export const Heading: UserComponent<HeadingProps> = ({
     selected: node.events.selected,
   }));
 
+  const safeLevel = sanitizeHeadingLevel(level);
   const elRef = useRef<HTMLElement | null>(null);
   const editedTextRef = useRef<string | null>(null);
 
@@ -59,7 +70,7 @@ export const Heading: UserComponent<HeadingProps> = ({
     }
   }, [text, selected]);
 
-  return React.createElement(level, {
+  return React.createElement(safeLevel, {
     ref: (ref: HTMLElement | null): void => {
       elRef.current = ref;
       if (ref) connect(drag(ref));
@@ -98,7 +109,7 @@ Heading.craft = {
 };
 
 (Heading as any).toHtml = (props: HeadingProps, _childrenHtml: string) => {
-  const tag = props.level || 'h2';
+  const tag = sanitizeHeadingLevel(props.level);
   const safeText = (props.text || '').replace(/</g, '&lt;').replace(/>/g, '&gt;');
   const styleStr = cssPropsToString(props.style);
   return { html: `<${tag}${styleStr ? ` style="${styleStr}"` : ''}>${safeText}</${tag}>` };