fix: make handleLookup async to fix bun next build error

fix: store session json in url/base64 modes, scan disk for missing sessions, support url mode late joining, and display available sessions on select screen

fix: store presentation slides in /data persistent storage and embed base64 fallback in session state

feat: Add persistent session export and slide URL generation for late-joining students

feat: full CORS support + OPTIONS preflight + interactive /api-docs reference page

Fix: Replace socket.io with SSE+REST, add 4-digit room codes, fix Join Session button

Add complete SSE backend with all API endpoints and detailed documentation