fix(desktop): pass log level through RUST_LOG

[alloutflo]

Summary

• pass the desktop-configured log level through the child process's RUST_LOG environment variable
• stop sending the unsupported --log-level argument to sensing-server
• add a command-construction regression test that checks both the environment and argument list

The sensing server already initializes its filter from RUST_LOG, so this preserves the setting while allowing the child process to start normally.

Validation

• cargo test --manifest-path v2/Cargo.toml -p wifi-densepose-desktop commands::server::tests::log_level_is_passed_through_rust_log --no-default-features
• git diff --check upstream/main...HEAD

This validates the Rust command boundary; it does not claim an end-to-end desktop or hardware run.

Closes #1516