Refactor WindowManager into partial classes

[PrzemyslawKlys]

Summary

• split monolithic WindowManager class into partial classes WindowManager.Actions and WindowManager.Layout
• convert WindowManager to file-scoped namespace

Testing

• dotnet build Sources/DesktopManager.sln -c Release
• dotnet test Sources/DesktopManager.sln -c Release (fails: Could not find 'mono' host)
• pwsh -NoLogo -NoProfile -Command ./DesktopManager.Tests.ps1 (fails: cmdlets not recognized)

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https://chatgpt.com/codex/tasks/task_e_687015c15a04832ebb280082430b22b5

[codecov]

Codecov Report

Attention: Patch coverage is 45.75163% with 83 lines in your changes missing coverage. Please review.

Project coverage is 53.23%. Comparing base (84fee87) to head (c6f9dc4).
Report is 8 commits behind head on v2-speedygonzales.

Files with missing lines	Patch %	Lines
Sources/DesktopManager/WindowManager.Actions.cs	40.00%	56 Missing and 7 partials ⚠️
Sources/DesktopManager/WindowManager.Layout.cs	58.33%	14 Missing and 6 partials ⚠️

Additional details and impacted files

@@                  Coverage Diff                  @@
##           v2-speedygonzales     #109      +/-   ##
=====================================================
+ Coverage              51.64%   53.23%   +1.59%     
=====================================================
  Files                     28       31       +3     
  Lines                   1340     1422      +82     
  Branches                 206      222      +16     
=====================================================
+ Hits                     692      757      +65     
- Misses                   573      583      +10     
- Partials                  75       82       +7     

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