Function resolve_chromium_executable 

pub fn resolve_chromium_executable(
    explicit_path: Option<PathBuf>,
) -> Option<PathBuf>

Resolves Chromium executable path for CBF applications.

Resolution order:

1. Explicit path (for example CLI argument)
2. CBF_CHROMIUM_EXECUTABLE environment variable
3. Path relative to current app bundle: ../Frameworks/Chromium.app/Contents/MacOS/Chromium

Returns None when no candidate can be resolved.