Implement centralized color scheme system with Sum of 6 greying

- Add ScoringColorScheme class with explicit text colors per scheme
- Implement traditional (vibrant) and lowSaturation (muted) color schemes
- Add disabled state colors for greying out inactive elements
- Refactor scoring_cell.dart and scoring_keypad.dart to use centralized colors
- Grey out Sum of 6 column when in 3-arrow end mode while maintaining layout
- Add comprehensive color scheme documentation and usage examples

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

lib/config/constants/_constants.md

@@ -0,0 +1,48 @@
+# Constants Directory
+
+This directory contains application-wide constants and configuration.
+
+## Files
+
+- **scoring_colors.dart**: Centralized color scheme system for all scoring-related UI components
+  - Provides multiple color schemes: `traditional` (vibrant) and `lowSaturation` (muted)
+  - Follows archery ring color conventions (gold, red, blue, black, white)
+  - Explicit text colors defined per scheme for intentional design control
+  - Disabled state colors for greying out inactive UI elements
+  - Single source of truth for all score-related colors with easy scheme switching
+  - Architecture ready for dark themes, high contrast, and colorblind-friendly schemes
+
+## Future Expansion
+
+Planned constants files:
+- `app_constants.dart`: General app configuration (timeouts, limits, defaults)
+- `layout_constants.dart`: UI spacing, sizing, and layout specifications  
+- `scoring_rules.dart`: Archery scoring rules and validation constants
+- `export_constants.dart`: File formats, templates, and export configurations
+
+## Usage
+
+```dart
+import 'package:archery_bookkeeper/config/constants/scoring_colors.dart';
+
+// Get background color for any score (uses current active scheme)
+Color bgColor = ScoringColors.getScoreBackgroundColor('X');
+
+// Get appropriate text color with automatic contrast calculation
+Color textColor = ScoringColors.getScoreTextColor('X');
+
+// Check if score should be bold
+bool isBold = ScoringColors.shouldUseBoldText('10');
+
+// Access available schemes
+ScoringColorScheme traditional = ScoringColors.availableTraditional;
+ScoringColorScheme lowSat = ScoringColors.availableLowSaturation;
+
+// Get disabled state colors
+Color disabledBg = ScoringColors.getDisabledBackground();
+Color disabledText = ScoringColors.getDisabledText();
+Color disabledBorder = ScoringColors.getDisabledBorder();
+
+// Switch schemes by changing _currentScheme in scoring_colors.dart:
+// static const ScoringColorScheme _currentScheme = lowSaturation;
+```