Add Set Scale and physical unit calibration

[yangfei]

Summary

• Add per-document PixelCalibration storage with pixel defaults.
• Add Analyze > Set Scale... for pixel width, pixel height, and unit.
• Use document calibration when measuring ROI/full image area and include Unit in the results table.
• Add core and app tests for calibrated area measurement.

Verification

• dotnet build ImageJCsharp.sln
• dotnet test tests/ImageJCsharp.Core.Tests/ImageJCsharp.Core.Tests.csproj
• dotnet test tests/ImageJCsharp.App.Tests/ImageJCsharp.App.Tests.csproj

Closes #15