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

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

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

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

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