Extend StyleDefaultsConfig to allow customization of default style values that were previously hardcoded as constants.
This enables global configuration without requiring stylesheet updates across all Graph instances.

New configurable properties:
  - Font settings: fontFamily, fontSize
  - Arrow settings: arrowSize, arrowSpacing, arrowWidth
  - Size settings: markerSize, imageSize, startSize
  - Rounding: roundingFactor, lineArcSize

All direct references to DEFAULT_* constants throughout the codebase have been replaced with StyleDefaultsConfig property access, allowing these defaults to be changed at runtime via the configuration object.