Fix #58: add autosave backup rotation and atomic write

[Nico-du-34]

Summary

This PR improves autosave reliability by adding backup rotation and atomic file writes.

Closes #58.

Changes

• Replace direct autosave writes with a safer flow:
  • Write JSON to a temporary file in the same directory
  • Rename existing autosave file to autosave.json.bak
  • Atomically move the temp file to autosave.json
• Add failure handling:
  • Clean up temporary files on error
  • Attempt to restore previous autosave from .bak if rotation happened but final replace failed
• Keep errors non-crashing through existing save-level exception handling and logging

Validation

• dotnet build grzyClothTool/grzyClothTool.csproj -v minimal
• Result: 0 errors