sitesmith: null-safe esc() across all toHtml + WorkingIndicator

Real-world AI output frequently sends mismatched prop names (e.g.
items vs features, cta object vs buttonText/Href). The toHtml functions
of section/form/sections-folder components each defined a local
esc = (s: string) => s.replace(...) that crashed when called with
undefined, taking the auto-save export with it.

Patched every local esc() to coerce non-strings:
  const esc = (s: any) => String(s ?? "").replace(...)
17 files touched; behavior unchanged for valid string inputs.

Also adds a WorkingIndicator (Claude Code-style spinner + rotating
phrase + elapsed seconds) shown in the modal footer while a generation
is in flight, replacing the disabled "Thinking..." placeholder.

craft/src/components/sections/HeroSimple.tsx

@@ -404,7 +404,7 @@ HeroSimple.craft = {
 /* ---------- HTML export ---------- */
 
 (HeroSimple as any).toHtml = (props: HeroProps, _childrenHtml: string) => {
-  const esc = (s: string) => s.replace(/</g, '&lt;').replace(/>/g, '&gt;');
+  const esc = (s: any) => String(s ?? "").replace(/</g, '&lt;').replace(/>/g, '&gt;');
   const bg = buildBackground(props);
   const justifyMap: Record<string, string> = { top: 'flex-start', center: 'center', bottom: 'flex-end' };