Implement pool filtering functionality for fixtures and ladders

[Copilot]

Adds comprehensive pool filtering support to the FIT mobile app, enabling users to filter fixtures and ladders by pool when pool information is available in the division API response.

Overview

The division API now includes pool information as part of stages:

{
  "stages": [
    {
      "title": "Pool Stage",
      "pools": [
        {"id": 122, "title": "Pool A"},
        {"id": 123, "title": "Pool B"}
      ],
      "matches": [
        {"id": 1, "pool_id": 122, "round": "Round 1", ...}
      ],
      "ladder_summary": [
        {"team": "team1", "pool_id": 122, ...}
      ]
    }
  ]
}

This PR implements the UI and logic to leverage this pool data for enhanced filtering and visual distinction.

Key Features

Smart Pool Filtering

• Hierarchical dropdown: Stage names as headers with pools as selectable options
• Conditional display: Pool filter only appears when pools exist in the data
• Intelligent interaction: Team and pool filters work together logically - selecting one clears the other, and team filtering is restricted to teams within the selected pool

Visual Pool Distinction

• 8 FIT brand colors: Pools rotate through Primary Blue, Success Green, Accent Yellow, Error Red, Purple, Light Blue, Orange, and Dark Green
• Enhanced round display: Shows "Round 1 - Pool A" format when pool information is available
• Color-coded indicators: Round badges use pool-specific colors for visual identification

Comprehensive Integration

• Fixtures tab: Pool filtering with color-coded match cards
• Ladder tab: Pool-based ladder filtering with proper stage grouping
• Backward compatibility: Gracefully handles divisions without pool data

Technical Implementation

Data Models

• Created Pool model with id/title fields and JSON serialization
• Enhanced Fixture model with optional poolId field
• Updated LadderEntry model with optional poolId field
• Extended LadderStage model to include pools collection

Color System

Extended the FIT color palette with pool-specific colors:

static const List<Color> poolColors = [
  primaryBlue,        // Pool A
  successGreen,       // Pool B  
  accentYellow,       // Pool C
  errorRed,           // Pool D
  Color(0xFF8E4B8A), // Pool E - Purple
  Color(0xFF4A90E2), // Pool F - Light blue
  Color(0xFFE67E22), // Pool G - Orange
  Color(0xFF27AE60), // Pool H - Dark green
];

UI Components

• MatchScoreCard: Enhanced to display pool information with color-coded round indicators
• FixturesResultsView: Added hierarchical pool filtering dropdown and intelligent filter interaction
• Ladder display: Pool-based filtering with maintained stage grouping

Benefits

1. Improved user experience: Users can quickly filter to relevant pool matches and standings
2. Visual clarity: Color-coded pools make it easy to distinguish between different pool competitions
3. Efficient navigation: Hierarchical filtering reduces cognitive load when browsing pool-based tournaments
4. Future-ready: Fully prepared for API responses containing pool data

Testing

• All new models compile without errors
• UI components maintain existing functionality when pools are not present
• Color system provides consistent visual differentiation across 8+ pools
• Filter interactions work as expected with proper state management

The implementation follows existing project patterns and maintains backward compatibility while adding powerful new filtering capabilities for pool-based competitions.

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