test WebRTC Direct v2 on Firefox and Safari #188

Follow-up to #186 and WithAutonomi/ant-node#220.

The browser client now uses the saorsa+webrtc+v2/ no-SDP-mutation connection profile and passes Chromium end-to-end with WebRTC-NoSdpMangleUfrag/Enabled/. ADR-0009 still requires interoperability proof in current Firefox and Safari before production promotion.

Acceptance criteria:

• Run direct-connect and bootstrap/upload/download flows in current Firefox from a real secure context.
• Run the same flows in current Safari from a real secure context.
• Record browser/OS versions and any SDP/ICE differences.
• Add sustainable automated coverage where the browser platform permits it.