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

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

fix: teacher can re-mark multiple correct answers; doubt slide popup uses SSE image or allSlides fallback

feat: multi-choice poll, poll tab ripple, Doubt Box with slide snapshot, teacher doubt popup + green resolve tick

feat: improve poll system - student notification, hide live results, correct/wrong feedback after teacher marks answer

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

fix: remove +1 slide number offset shown to students

Add Slides tab with all slides + current marker; fix poll history, correct answer marking; fix real vote data

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

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

Fix: API docs page - remove invalid html/body tags, use proper React/Next.js syntax

Add API Documentation page at /api-docs with full SSE endpoint docs

Fix: Date.now().slice error - convert to string before slicing

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