chore: completely remove base64 payloads to fix buffer explosion and silent socket crashes, rely strictly on lightweight xethub urls

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: slides now broadcast imageBase64 to students; polls always work (auto-end previous); endPoll implemented

Add complete SSE backend with all API endpoints and detailed documentation