sitesmith: narrow CANVAS_TYPES to just Container

The canonical Craft.js state from real saves shows that layout shells
(Section, BackgroundSection, HeroSimple, FeaturesGrid, ColumnLayout,
CTASection, FormContainer, Navbar, Footer) all serialize with
isCanvas:false. Only Container instances are canvases. The shells use
internal <Element canvas id="..."> linkedNodes for their drop targets.

Our previous CANVAS_TYPES set claimed all those shells were canvases,
which made Craft.js's toNodeTree walker hit an uncaught Invariant —
the shell asserted "I'm a canvas" but its render ignores data.nodes,
so the walker would chase phantom children.

craft/src/state/PageContext.tsx

@@ -4,16 +4,13 @@ import { PageData } from '../types';
 import { SerializedTreeNode } from '../types/sitesmith';
 import { useSiteDesign, SiteDesign } from './SiteDesignContext';
 
-/** Layout components that accept children. Must match the .craft.rules.canMoveIn
- * config of each component. Leaf components (Heading, TextBlock, ButtonLink, …)
- * are NOT canvases and must serialize with isCanvas:false or their rendered
- * content gets swallowed by an empty drop-target wrapper. */
-const CANVAS_TYPES = new Set<string>([
-  'Container', 'Section', 'ColumnLayout', 'BackgroundSection',
-  'HeaderZone', 'FooterZone',
-  'HeroSimple', 'FeaturesGrid', 'CTASection',
-  'FormContainer', 'Navbar', 'Footer',
-]);
+/** Only `Container` instances are "real" canvases in serialized state — they
+ * directly render whatever is in node.data.nodes. Layout-shell components
+ * (Section, HeroSimple, FeaturesGrid, ColumnLayout, CTASection, etc) use
+ * Craft.js <Element canvas id="…"> linkedNodes internally; their own
+ * isCanvas must be FALSE or Craft.js's toNodeTree walker trips an Invariant
+ * because the shell claims to be a canvas but its render ignores `nodes`. */
+const CANVAS_TYPES = new Set<string>(['Container']);
 
 interface PageContextValue {
   pages: PageData[];