Optimize selective export implementation for performance and maintain…

[speleobrad]

Refactor and optimize the selective survey export feature with significant improvements:

Performance Optimizations:

• Add SelectionStats class for efficient single-pass selection counting
• Optimize toggleSelectAll() to avoid redundant selection state checks
• Refactor SectionCard widget into smaller, focused methods for better rendering
• Implement efficient bulk selection operations with setSelectionForSections()

Code Quality Improvements:

• Extract DataToolbar widget methods for better maintainability and readability
• Add comprehensive error handling with user-friendly snackbar notifications
• Refactor export handlers in main.dart to eliminate code duplication
• Create reusable helper methods for export operations

New Features:

• Add toggleSelection() method to Section model for convenient state toggling
• Implement SelectionStats with comprehensive selection state information
• Add robust error handling for all export operations with visual feedback

Testing Enhancements:

• Expand test coverage to include new SelectionStats functionality
• Add tests for bulk selection operations and performance optimizations
• Verify all optimization features work correctly