Add graceful shutdown handling for long-running operations

[dapi]

Problem

The --scan command iterates through the entire port range (default 1000 ports) and can take significant time. If interrupted (Ctrl+C), there is no signal handling:

• Partial data may be written
• No cleanup performed
• User gets no feedback about interruption

Current Behavior

$ port-selector --scan
# User presses Ctrl+C during scan
# Process terminates immediately
# Allocations file may be in inconsistent state

Proposed Solution

Add signal handling for SIGINT and SIGTERM:

func runScan(...) error {
    ctx, cancel := context.WithCancel(context.Background())
    
    sigChan := make(chan os.Signal, 1)
    signal.Notify(sigChan, syscall.SIGINT, syscall.SIGTERM)
    
    go func() {
        <-sigChan
        fmt.Fprintln(os.Stderr, "\nInterrupted, saving partial results...")
        cancel()
    }()
    
    // Use ctx in scan loop
    for port := start; port <= end; port++ {
        select {
        case <-ctx.Done():
            return savePartialResults()
        default:
            // scan port
        }
    }
}

Acceptance Criteria

• Handle SIGINT/SIGTERM in --scan
• Save partial results on interruption
• Print message about interruption
• Consider applying to other long-running operations

Priority

Low - edge case, but improves robustness