feat: add ARC command with multi-input construction flows and i18n

[mlightcad]

Summary

This PR introduces a new ARC command in cad-simple-viewer, including interactive jig preview, multiple arc-construction workflows, and localized prompts/messages in English and Chinese.

What Changed

• Added AcApArcCmd and registered arc in the command manager.
• Exported the new command from src/command/index.ts.
• Implemented arc creation flows in AcApArcCmd:
  • Start -> Second -> End (3-point arc)
  • Start -> Center -> End/Angle/Chord Length
  • Start -> End -> Center/Angle/Direction/Radius
  • Center -> Start -> End/Angle/Chord Length
• Added dynamic preview jig behavior for arc creation.
• Added invalid-geometry warnings for unsupported/degenerate inputs.
• Added direction-flip handling via input toggle (Ctrl) for preview and final commit consistency.
• Added i18n entries:
  • Command description (en / zh)
  • Arc prompts, keywords, and validation messages (en / zh)