Reorganize codebase into modular architecture

The backend has been refactored from a single monolithic server.js file into a modular structure with separate concerns: routing (router.js), AI integrations (src/ai.js, parsers for Claude/Codex/Copilot), features (analytics, history, notifications), and individual route handlers. The frontend also gained an alert drawer UI for viewing all active alerts across sessions in one place, with improved visual indicators (warning icons) on session items.