v1.5.0

v1.5.0: PID lockfile-based single-instance enforcement

Major improvements:
- Replaced unreliable Windows global Mutex with PID lockfile
- Automatic stale lockfile detection and cleanup
- Process PID validation ensures accuracy
- Fixes: duplicate monitor processes on startup
- Fixes: mutex permission errors on some systems
- More inspectable and portable

Testing results:
 Fresh install starts monitor
 Second launch detects existing monitor
 Only 1 process runs (not duplicates)
 Lockfile contains correct PID
 Stale lockfiles automatically cleaned