feat: add centralized color constants

- Create AppColors class with semantic color names
- Define 7 core colors (background, primary, textSecondary, etc.)
- Single source of truth for theming
- Improves maintainability and consistency

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

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

Changed files
+31
lib
constants
+31
lib/constants/app_colors.dart
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import 'package:flutter/material.dart';
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/// Application color constants
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///
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/// Centralized color definitions for consistent theming.
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/// Using semantic names improves maintainability.
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class AppColors {
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// Private constructor to prevent instantiation
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AppColors._();
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/// Primary background color - dark navy
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static const background = Color(0xFF0B0F14);
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/// Primary accent color - orange/coral
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static const primary = Color(0xFFFF6B35);
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/// Secondary text color - light gray/blue
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static const textSecondary = Color(0xFFB6C2D2);
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/// Border color - dark gray
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static const border = Color(0xFF2A2F36);
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/// Secondary background color - slightly lighter than main
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static const backgroundSecondary = Color(0xFF1A1F26);
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/// Loading indicator color - medium gray
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static const loadingIndicator = Color(0xFF484F58);
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/// White color for primary text
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static const textPrimary = Colors.white;
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}