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/panels/sitesmith/SitesmithModal.tsx

@@ -8,6 +8,7 @@ import { UpgradeBanner } from './UpgradeBanner';
 import { ScopeConfirmDialog } from './ScopeConfirmDialog';
 import { MessageList } from './MessageList';
 import { ChatInput } from './ChatInput';
+import { WorkingIndicator } from './WorkingIndicator';
 import { SitesmithResponse } from '../../types/sitesmith';
 
 interface Props { onClose: () => void; }
@@ -76,11 +77,15 @@ export const SitesmithModal: React.FC<Props> = ({ onClose }) => {
             : <MessageList messages={messages} />}
         </div>
         <div style={footer}>
-          <ChatInput
-            disabled={!canChat || busy}
-            placeholder={!canChat ? 'Upgrade your plan to use Sitesmith' : busy ? 'Thinking…' : 'Describe what you want…'}
-            onSend={handleSend}
-          />
+          {busy ? (
+            <WorkingIndicator />
+          ) : (
+            <ChatInput
+              disabled={!canChat}
+              placeholder={!canChat ? 'Upgrade your plan to use Sitesmith' : 'Describe what you want…'}
+              onSend={handleSend}
+            />
+          )}
         </div>
         <ScopeConfirmDialog
           open={!!pendingReplace}