Guard snapshot process signals in GUI

[Ajeem-git]

Summary:
Prevent a GUI crash when pause, resume, or stop is clicked after the snapshot process has already disappeared on a slow system.

What changed:

• route snapshot SIGSTOP / SIGCONT / SIGKILL through a guarded helper
• refresh backup state if the snapshot PID is already gone instead of crashing with ProcessLookupError
• keep the stop action behavior the same when the process is still alive

Testing:

• python3.11 -m py_compile qt/app.py

Closes #1604

[buhtz]

Hello Ajeem,

thank you for your contribution and taking time to improve Back In Time. I appreciate it.

On a quick initial review your PR looks good. I'll review it in detail later.

Please note that currently I have to put all PRs on hold, because the Back In Time code base is in a transition phase. I am rewriting the mounting subsystem from scratch (PR #2449). Since the components are not well isolated, this affects nearly every file. To avoid complex merge conflicts, all other PRs are temporarily on hold.

Best wishes,
Christian