feat: let Mixed listen on the LAN

[harryisfish]

Summary

• Add mixed_lan so Mixed can bind 0.0.0.0 and :: and accept LAN clients.
• The controller, DNS listener, and System Extension SOCKS listeners stay on loopback. Mixed still has no inbound password.
• The switch is on 设置 → 显式代理（Mixed） and myproxyctl mixed-lan on|off. Apply or connect activates it.

Test plan

• cargo test --lib -- mixed_lan_opens_mixed_only compile_emits_mixed_listener missing_inbound_modes_default_to_rule
• cargo check --bin myproxyctl
• On a connected Mac, turn 对局域网开放 on, apply, and connect from another LAN device to <ip>:7891
• Confirm 127.0.0.1:7998 and the extension SOCKS ports still refuse LAN connections

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