Improve H264 streaming fallback

[DjDeveloperr]

Summary

• Adds the H264 WebSocket fallback and transport picker while keeping WebRTC as the primary live path.
• Removes the MJPEG fallback work and keeps stream stats/config traffic on the active stream channels instead of REST polling spam.
• Speeds first-frame and stream-switch paths by reusing valid cached sync keyframes, shortening local ICE waits, and avoiding client-side canvas/video remount reconnects.

Testing

• cargo fmt --manifest-path server/Cargo.toml --check
• cargo clippy --manifest-path server/Cargo.toml --all-targets -- -D warnings
• cargo test --manifest-path server/Cargo.toml
• prettier --check .
• PATH="client/node_modules/.bin:$PATH" tsc --noEmit from client/
• node node_modules/vitest/vitest.mjs run from client/
• node node_modules/vite/bin/vite.js build from client/
• scripts/build-cli.sh
• Chrome smoke test on 127.0.0.1:4311: H264 WS first canvas frame, explicit WebRTC first video frame, stale stream=mjpeg falls back without MJPEG endpoint requests, and no client-stream-stats/stream-quality POST spam.