- Created generic ErrorPage.js with errorCode prop
- Centralized error messages in ERROR_MESSAGES dictionary
- Updated App.js to use ErrorPage for all error routes
- Updated ErrorBoundary.js to use new ErrorPage component
- Removed duplicate files: 403Page.js, 404Page.js, 500Page.js, 502Page.js, 503Page.js
- Fixed 403/404 routing: protected routes show 403, unknown routes show 404
- Error pages now vertically centered with min-height: 100vh
