Improve single-instance safety and browser process handling

[geovanibruno]

Summary

This pull request improves WinLock's process handling, single-instance
behavior, browser shutdown flow, and compatibility with virtual environments.

Motivation

Browsers commonly run several processes using the same executable name.
Treating each process as a separate protected application can cause duplicate
lock screens during the same monitoring cycle.

The previous single-instance implementation also searched for and terminated
Python processes based on their command line. In virtual environments, the
launcher and runtime may appear as separate Python processes, which can cause
WinLock to terminate its own active instance.

Finally, immediately killing every browser process can make browsers interpret
Cancel as an unexpected shutdown and display crash-recovery or troubleshooting
prompts on the next launch.

Changes

• Replace command-line-based Python process termination with a named Windows
  mutex.
• Deduplicate protected applications by executable name during each process
  scan.
• Request a normal browser shutdown through WM_CLOSE before using a bounded
  forced-termination fallback.
• Handle missing process names and expected psutil process-access errors
  explicitly.
• Preserve the executable path correctly when registering auto-start,
  including virtual-environment installations.
• Add unit tests for:
  • multi-process application deduplication;
  • case-insensitive executable names;
  • missing process names;
  • custom targets such as Zen Browser.
• Document the test behavior and how to configure additional applications.

Compatibility

• The default PIN remains 1234.
• The default protected applications remain unchanged.
• Zen Browser is documented and tested as an example of a custom target, but
  is not enabled by default.
• No machine-specific paths or user-specific configuration are included.

Testing

python -m unittest discover -s tests -v

Result:

Ran 3 tests in 0.001s

OK

The tests use simulated process information and do not start or terminate real
browsers.

Licensing note

The repository currently does not include an explicit license. No license is
added in this pull request because that choice should be made by the original
copyright holder.