fix: add missing signal.Stop in suspendProcess to prevent signal channel leak (Closes #1673) (#1674)

Each Ctrl+Z suspend call registered a SIGCONT channel via signal.Notify
but never called signal.Stop after receiving the signal. This caused a
signal channel leak on every suspend/resume cycle.

All other signal handlers in the codebase (handleSignals in tea.go and
listenForResize in signals_unix.go) correctly defer signal.Stop.

Signed-off-by: Cocoon-Break <54054995+kuishou68@users.noreply.github.com>