v0.2.3-pre - WebRTC peer-to-peer transport

Aurora 0.2.3-pre adds a peer-to-peer WebRTC message transport so conversations can connect across carrier NAT.

What's new

• Messages connect over a WebRTC data channel (ICE + self-hosted STUN, no TURN). The rendezvous only helps the two devices find each other, then falls off the data path.
• IPv6 first, IPv4 as fallback, with direct TCP as a same-network fast path.
• The chat header shows the live connection state (Connecting, Connected peer-to-peer, or an honest could-not-connect message).
• Self-hosted STUN runs on the existing rendezvous server (STUN only, no relay).

Known limits

• Two peers both on symmetric carrier NAT with no IPv6 still cannot connect directly; a peer-relay (ShadowMesh) for that case is future work.
• Live two-device connection is best verified on physical phones on different networks.

Pre-alpha, experimental software. Do not trust it with anything critical yet. Sideload only (Android).