fix: remove os.Exit from mail watch signal handler

[luojiyin1987]

Problem

The mail watch command uses os.Exit(0) in its signal handler to terminate the program when receiving Ctrl+C. This approach has several drawbacks:

1. Not testable - os.Exit terminates the process immediately, preventing unit tests from verifying shutdown behavior
2. Bypasses cleanup - Deferred cleanup functions in the call stack are not executed
3. Unidiomatic - CLI tools should use context cancellation for graceful shutdown

Solution

Replace os.Exit(0) with a graceful shutdown mechanism using:

• context.WithCancel to propagate cancellation to the WebSocket client
• A shutdownBySignal channel to coordinate between signal handler and main goroutine
• Proper defer signal.Stop(sigCh) for resource cleanup

Changes

• shortcuts/mail/mail_watch.go: Remove os.Exit(0), add context-based shutdown coordination