Fix Electron production CSS not loading under file://.

Strip crossorigin from the built index so stylesheets apply in the packaged app, and use a relative manifest path for the same reason.

src/app/components/message/RenderBody.tsx

@@ -6,8 +6,6 @@ import { MessageEmptyContent } from './content';
 import { sanitizeCustomHtml } from '../../utils/sanitize';
 import { highlightText, scaleSystemEmoji } from '../../plugins/react-custom-html-parser';
 import { isMxEmoticonOnlyHtml } from '../../utils/regex';
-// Ensure jumbo emoji global styles are always in the bundle.
-import './layout/layout.css';
 
 type RenderBodyProps = {
   body: string;