fix(engine): choose a reachable address for QR pairing #1925

The default gateway launched by /remote binds 127.0.0.1. A phone can scan the QR but cannot connect to that address; current setup asks the user to discover a LAN address and restart the gateway manually. This prevents the requested scan-and-connect flow.

Evidence: RemoteCommands::Serve defaults to 127.0.0.1; GatewayConnector launches remote serve without a bind unless the user configured one. Existing interface discovery already provides local addresses. The user reports pairing is too difficult and wants the host address embedded automatically.

Acceptance criteria:

• /remote and remote serve automatically choose a usable Wi-Fi/Ethernet host address on a connected machine.
• QR and manual pairing show that same actual listener address, never a router or wildcard address.
• Explicit loopback, LAN, NetBird and advertised-address overrides continue working.
• Selection handles multiple interfaces and a missing network predictably, with regression coverage.
• Document same-network scanning and the remaining physical-iPhone acceptance check; do not claim arbitrary internet connectivity through NAT.