fix: honor proxy settings for relay websockets

[M4n5ter]

Summary

• add a shared proxy-aware WebSocket connector that reads HTTP_PROXY, HTTPS_PROXY, ALL_PROXY, and NO_PROXY
• discover static Windows and macOS system proxy settings, with HTTP CONNECT and SOCKS4/5 tunnel support
• normalize Windows per-protocol ProxyServer lists before building the shared matcher
• keep localhost and loopback relay URLs direct so the default local relay remains unchanged
• use the shared connector across Desktop relay, pairing, huddle audio, buzz-acp, buzz-pairing-cli, and buzz-ws-client paths
• keep proxy credentials out of connection errors and preserve DNS error classification through connector context

Related issue

Fixes #3281.

Testing

• cargo fmt --all -- --check
• cargo fmt --manifest-path desktop/src-tauri/Cargo.toml --all -- --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test -p buzz-ws-client
• cargo test -p buzz-acp
• cargo check -p buzz-pairing-cli -p buzz-cli
• cargo deny check
• cargo zigbuild -p buzz-ws-client --lib --target x86_64-pc-windows-gnu

The connector regression tests exercise direct loopback bypass, authenticated HTTP CONNECT without local target DNS, SOCKS5h remote DNS, proxy credential redaction, Windows per-protocol proxy parsing, and nested DNS error classification. Desktop Tauri compile/tests were not run locally because this WSL host lacks the required GTK/WebKit development packages.