Add configurable horizontalBias and verticalBias to GraphView

[rashidotm]

Summary

• Adds horizontalBias and verticalBias parameters to GraphView and GraphView.builder constructors
• Allows consumers to control where the graph is positioned within the viewport (e.g., verticalBias: 0.3 shifts the graph upward)
• Defaults to 0.5 (centered), preserving existing behavior

Closes #146

Test plan

• flutter analyze — no new warnings or errors
• Run the example app — default behavior unchanged (bias = 0.5)
• Test with non-default values (e.g., verticalBias: 0.3) to confirm graph shifts as expected

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