feat: attribute gateway clients by process

[kev1n77]

Summary

• resolve Gateway client PIDs from exact loopback TCP endpoint matches on Windows and macOS
• carry the optional PID through capture policy, encrypted storage, desktop API, and Inspector without persisting socket endpoints
• degrade all attribution failures to unknown and keep Proxy PID attribution explicitly pending

Verification

• cargo fmt --all -- --check
• cargo test --workspace --all-targets
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --manifest-path apps/desktop/src-tauri/Cargo.toml --all-targets
• cargo clippy --manifest-path apps/desktop/src-tauri/Cargo.toml --all-targets -- -D warnings
• npm run desktop:check
• npm run desktop:build