macOS Desktop backend processes can still miss Apple Silicon Homebrew paths even after adding Hermes-managed Node and venv bins. That leaves `/codex-runtime on` unable to find a Homebrew-installed `codex` binary at `/opt/homebrew/bin/codex`.
Add a small testable backend env helper that builds the dashboard subprocess environment in one place. It prepends Hermes-managed Node and venv bins, appends missing POSIX sane PATH entries individually, preserves caller precedence without duplicates, and keeps Windows PATH casing/delimiters intact.
Wire both source-checkout and active-install backend descriptors through the helper, and add Node regression coverage to the desktop platform test suite.
