refactor: use System.Threading.Lock for the remaining monitor locks

[laurentiu021]

Summary

Completes the System.Threading.Lock migration. ActivityLogService was already
converted; this brings the remaining six monitor locks to the same idiom. No
behavior change — lock(Lock) lowers to EnterScope()/Dispose(), identical
mutual exclusion to a lock(object) monitor.

Changes

private readonly object _x = new() → private readonly Lock _x = new() in:

• Services/AppAlertService.cs — _watcherLock
• Services/IconExtractorService.cs — _evictionLock (static)
• Services/PingMonitorService.cs — _stateLock
• Services/SystemInfoService.cs — _cacheLock
• Services/TracerouteMonitorService.cs — _stateLock
• ViewModels/ConsoleViewModel.cs — _gate

Why Lock

The dedicated lock type makes "this field exists only to be locked on" explicit at
the type level, lets the analyzer flag accidental Monitor/Wait/Pulse misuse,
and uses the lighter EnterScope lock path. System.Threading is already in the
project's implicit usings, so no using change is needed.

Verification

• Every field verified to be used only as a lock() target — no Monitor.Wait/Pulse,
  not passed as an argument, not locked from outside the declaring type.
• Build: main + Tests + IntegrationTests all 0 warnings / 0 errors.
• The 13 lock() statement sites compile unchanged against the new type.

Behavior identical (Protocol Q). refactor: → no release.