[Feature] Add onSegmentPressed callback to detect pie chart segment taps

[GeekTree0101]

Summary

This pull request adds a new callback onSegmentPressed to the PieChart widget, allowing developers to detect and respond when a specific pie segment is tapped.

Key Changes

• Introduced onSegmentPressed(String key) callback in the PieChart constructor.
• Wrapped chart area in a GestureDetector to capture tap positions.
• Implemented angle calculation logic to map touch positions to corresponding pie segments.
• Ensured compatibility with initialAngleInDegree and custom segment counts.

Use Case

This enables interactive chart behavior such as:

• Highlighting selected segments
• Showing tooltips or detailed views
• Navigating based on chart input

Example

PieChart(
  dataMap: {
    'Food': 40,
    'Rent': 30,
    'Travel': 30,
  },
  onSegmentPressed: (key) {
    print('Tapped segment: $key');
  },
)