fix(recording): preserve high-DPI screencast frames

Summary

• Match recording screencast frames to the initial PNG's device-pixel dimensions.
• Restore the CSS viewport content size when recording stops.

Closes #2195.

Validation

• cargo fmt --check
• cargo test --quiet -- --test-threads=1
• cargo test e2e_recording_screencast_preserves_device_scale_factor -- --ignored --nocapture
• cargo clippy -- -D warnings