Make ShareScreenRTC serverless: QR-based offer and compact response tokens

[espmaniac]

Motivation

• Replace the previous Python-based signaling workflow with a serverless flow that shares the offer via QR and accepts a typed short code plus a compact response token from the viewer.
• Simplify usage for static hosting or direct file open, removing the requirement to run server.py or a separate HTTP server.
• Improve UX for mobile viewers by embedding the offer in a QR URL and providing a compact, compressed response token that the broadcaster pastes in.

Description

• Rewrite of index.html to remove server polling and server endpoints, add serverless flows (startBroadcast, joinBroadcast) and UI for site code, response code and compact response token.
• Implement compact token encoding/decoding utilities including b64urlEncode/b64urlDecode, optional gzip compression via CompressionStream, and encodeCompactPayload/decodeCompactPayload.
• Change peer-connection logic: offerer flow generates a QR URL containing a base64url-encoded offer, viewer flow creates a short verifier code (SHA-256-derived) and returns a compact token containing the answer and verifier, and the broadcaster verifies site code and response code before applying the answer.
• Update styling and page copy and simplify README.md to document the new serverless flow, run options (Local file or Static host URL) and notes about local-network RTCPeerConnection({ iceServers: [] }) behavior.

Testing

• No automated tests were added or executed for this change.

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Codex Task