refactor: separate application UI state owners

src/reasonableDefaults.test.mjs

@@ -1,3 +1,4 @@
+import { readShellSource } from './testSupport/readShellSource.mjs';
 import { expandApplicationHtml } from '../build/application-html.js';
 // src/reasonableDefaults.test.mjs
 //
@@ -43,7 +44,7 @@ import {
 import { ShareLinkManager } from './sharelink.js';
 
 // Follow the UI wiring and its extracted preset definitions.
-const uiSource = fs.readFileSync(new URL('./ui/applicationShell.js', import.meta.url), 'utf8')
+const uiSource = readShellSource()
   + '\n' + fs.readFileSync(new URL('./ui/visualPresets.js', import.meta.url), 'utf8');
 const indexHtml = expandApplicationHtml(fs.readFileSync(new URL('../index.html', import.meta.url), 'utf8'));
 const shareSource = fs.readFileSync(new URL('./sharelink.js', import.meta.url), 'utf8');