Project units and New project defaults

[trailcode]

Summary

• Add File -> Project units (Inches / Millimeters) for on-the-fly display/input mapping; model space stays inch-scaled and geometry is not rewritten.
• Persist projectUnit in .ezy; older files default to Inches.
• Add Settings -> New project defaults (default unit + 2D view width/height shown in that unit; stored as inches) applied by File -> New.
• Wire length I/O (dims, Tab distance, add dialogs, chamfer/fillet, grid Settings, scripts dim()) through get_display_to_model_scale().

Test plan

• Build Debug EzyCad / EzyCad_tests
• Run EzyCad_tests.exe --gtest_filter=Sketch_test.ProjectUnit_displayConversionAndJsonRoundTrip
• File -> Project units: switch Inch/mm; confirm dims remap (1 in -> 25.4 mm) and geometry unchanged
• Save/reload .ezy; confirm projectUnit round-trips; open old file without field -> Inches
• Settings -> New project defaults: set Millimeters and framing; File -> New applies unit and framing
• Grid Settings step/padding show unit suffix matching open project
• File -> Export unit picker still independent
• Spot-check docs: usage.md, usage-settings.md, CHANGELOG

Closes #212

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