feat: Add configurable win rules (K-in-a-row and no-moves detection)

[dhanjit]

Summary

This PR adds two new configurable game rules to enhance gameplay flexibility:

1. Configurable Win Line Length (K-in-a-row)

   • Players can configure how many consecutive balls are needed to win (3 to N)
   • Smart default: N-in-a-row for N×N grid (maintains backward compatibility)
   • UI dropdown allows easy configuration
   • Example: Play 3-in-a-row on a 5×5 grid for faster games

2. No Valid Moves = Loss

   • Player automatically loses if they have no legal moves available
   • Always enabled (no toggle needed)
   • Detects immobilization during movement phase
   • Shows distinct win message for immobilization victories

Implementation Details

Core Game Logic (web/js/game.js)

• ✅ Add winLineLength parameter to constructor with validation (3 ≤ K ≤ N)
• ✅ Implement sliding window algorithm via checkLineForKConsecutive() method
• ✅ Refactor checkWinner() to use sliding window for K-in-a-row detection
• ✅ Add hasValidMoves() method to detect player immobilization
• ✅ Update getState() to expose winLineLength

Game Flow Integration (web/js/main.js)

• ✅ Read winLineLength config from UI dropdown in initGame()
• ✅ Add no-moves check after each successful move in movement phase
• ✅ Handle immobilization win condition for both PvP and PvAI modes

UI Updates (web/index.html, web/js/ui.js)

• ✅ Add Win Condition dropdown with dynamic options based on grid size
• ✅ Implement showWinnerByImmobilization() for distinct win message
• ✅ Update rules section to explain new victory conditions
• ✅ Add JavaScript to dynamically populate dropdown when grid size changes

AI Adaptation (web/js/ai.js)

• ✅ Mirror game.js sliding window logic in checkWinnerFromState()
• ✅ Refactor evaluateLine() to use sliding window for variable K evaluation
• ✅ Add immediate win detection in getBestMovement() for optimization
• ✅ Add no-moves detection to minimax() algorithm
• ✅ Fix cloneGameState() to preserve winLineLength property

Testing (tests/ai.test.js)

• ✅ Fix "detects winning movement opportunity" test setup
• ✅ Add Player 1 balls to create valid movement phase scenario
• ✅ Ensure tests verify AI correctly detects K-in-a-row wins

Test Results

All 130 tests passing:

• ✅ 100 game logic tests (including new K-in-a-row and no-moves scenarios)
• ✅ 29 AI tests (including immediate win detection)
• ✅ Integration tests for PvP and PvAI modes

Backward Compatibility

✅ Fully backward compatible: Default behavior (K=N) matches original game

• Existing games continue to work exactly as before
• New features are opt-in via UI dropdown
• No breaking changes to game logic or API

Usage Example

// Create a game with custom win condition
const game = new PaperballsGame(
  5,           // 5×5 grid
  'short',     // 8-way movement
  3            // 3-in-a-row to win
);

Screenshots

N/A - UI changes are functional (dropdown menu additions)

Related Issues

Addresses user request for configurable win rules to support different game variants and skill levels.