Add monitor power events

[PrzemyslawKlys]

Summary

• extend MonitorWatcher with monitor power notifications
• handle WM_POWERBROADCAST in new PowerBroadcastWindow
• expose MonitorPoweredOff and MonitorPoweredOn events
• add power notification tests
• update MonitorWatcher example

Testing

• dotnet build Sources/DesktopManager.sln -c Release
• dotnet test Sources/DesktopManager.sln -c Release (fails: Could not find 'mono' host)

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https://chatgpt.com/codex/tasks/task_e_686d44c01e0c832e9455d4853849f9f1

[codecov]

Codecov Report

Attention: Patch coverage is 91.22807% with 5 lines in your changes missing coverage. Please review.

Project coverage is 50.69%. Comparing base (1092e4e) to head (417a80c).
Report is 31 commits behind head on v2-speedygonzales.

Files with missing lines	Patch %	Lines
Sources/DesktopManager/MonitorWatcher.cs	91.07%	2 Missing and 3 partials ⚠️

Additional details and impacted files

@@                  Coverage Diff                  @@
##           v2-speedygonzales     #100      +/-   ##
=====================================================
+ Coverage              49.04%   50.69%   +1.64%     
=====================================================
  Files                     21       26       +5     
  Lines                   1150     1298     +148     
  Branches                 177      200      +23     
=====================================================
+ Hits                     564      658      +94     
- Misses                   530      571      +41     
- Partials                  56       69      +13     

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