fix: tolerate missing Mihomo proxy members

[d0zingcat]

Summary

Mihomo can occasionally return a policy group whose all list references proxies omitted from the /proxies response. The TUI previously assumed those members were always present and panicked while building the proxy tree.

This change makes the proxy tree resilient to that inconsistent runtime response:

• skip group members that are absent from /proxies
• leave the group unselected when its reported current member is unavailable
• omit groups with no remaining members, preventing empty-list expansion panics
• add regression coverage for missing members, missing current nodes, empty groups, and normal selection

Verification

• rustfmt --check clashctl/src/ui/components/proxy/tree.rs
• cargo +stable test -p clashctl ui::components::proxy::tree::tests
• cargo +stable build --release
• Opened the TUI successfully against a live Mihomo controller

Notes

The repository currently uses a floating nightly toolchain that is incompatible with its locked io-lifetimes 0.7.3 dependency. The checks above were run with the installed stable toolchain; no toolchain or dependency changes are included in this PR.