Function start_execution 

pub fn start_execution(
    app: &mut AppState,
    items: &[PackageItem],
    action: PreflightAction,
    header_chips: PreflightHeaderChips,
    password: Option<String>,
)

What: Start command execution by transitioning to PreflightExec and storing ExecutorRequest.

Inputs:

• app: Mutable application state
• items: Packages to install/remove
• action: Install or Remove action
• header_chips: Header chip metrics
• password: Optional password (if already obtained from password prompt)

Details:

• Transitions to PreflightExec modal and stores ExecutorRequest for processing in tick handler