feat: implement two-passage system to avoid OpenRouter rate limits

fix: show welcome overlay immediately on DOM load, before any async operations

improve: enhance welcome overlay styling and timing

feat: add welcome overlay for first-time user onboarding

fix: use complete app.js with full UI logic

fix: resolve all console errors and CSP violations

fix: remove redundant bookDataService initialization

fix: resolve module import errors and CSP issues

feat: minimal deployment with essential JavaScript modules and complete CSS