fix: remove authnMessages export from index.ts

The old App.messages pattern included apps exporting their messages via
\`src/index.ts\`. There is no reason for apps to export this. The file
only exists so the app can run as a site with \`npm run dev\`. With
frontend-base's i18n pipeline, translations are handled at the site
level via \`site.i18n\`.

App-only testing didn't catch this because the webpack fallback plugin
correctly resolves the missing messages file when \`npm run dev\` is
running the app as a site from the repo root (\`src/i18n/\` context).
The broken export only surfaces when the compiled \`dist/\` is consumed
by a site.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: 2 people

src/index.ts

@@ -1,3 +1,2 @@
 export { default as authnApp } from './app';
 export { default as authnRoutes } from './routes';
-export { default as authnMessages } from './i18n';