Add data-loaded attribute for scrns wait, fix og-ward screenshot

Add `data-loaded` DOM attribute to root div when data fetch completes,
and use `[data-loaded]` as the scrns selector. This makes `waitForSelector`
block until GeoJSON is loaded, fixing the og-ward screenshot where ward
geometry appeared unloaded with a fixed sleep timer.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: ryan-williams

www/public/cast.gif

@@ -5,7 +5,7 @@ const wardView = '?v=40.7310-74.0471+11.8+57-2'
 export default {
   host: 3201,
   output: 'public',
-  selector: '#root',
+  selector: '[data-loaded]',
   screenshots: {
     'og-lot': {
       query: `${defaultView}&agg=lot&sel=14507-1`,

www/public/og-ward.png

@@ -650,7 +650,7 @@ export default function App() {
   }
 
   return (
-    <div style={{ width: '100vw', height: '100vh', WebkitTouchCallout: 'none' }} onContextMenu={e => e.preventDefault()}>
+    <div style={{ width: '100vw', height: '100vh', WebkitTouchCallout: 'none' }} onContextMenu={e => e.preventDefault()} {...(!loading && { 'data-loaded': '' })}>
       <DeckGL
         viewState={viewState}
         onViewStateChange={({ viewState: vs }) => {