Add comprehensive unit tests with Vitest and HappyDom

[Copilot]

This PR implements comprehensive unit testing for the flood-it game using Vitest and HappyDom as requested in the issue.

Changes Made

🧪 Test Infrastructure

• Added Vitest: Modern test runner with excellent TypeScript and ES modules support
• Added HappyDom: Fast DOM environment for component testing
• Added Testing Library: @testing-library/preact for component testing utilities
• Configured Vitest: Updated vite.config.ts with test configuration and setup files

🔧 Code Refactoring for Testability

• Extracted utility functions from src/app.tsx to src/utils.ts:
  • createSeed(width, height) - generates random game seeds
  • parseSeed(seed) - parses and validates/fixes seed strings
  • flood(x, y, target, replacement, grid) - flood fill algorithm
  • colors - game color constants
• Updated imports in src/app.tsx to use the new utils module

📋 Test Coverage (23 tests total)

src/utils.test.ts (15 tests)

• ✅ Color array validation
• ✅ Seed generation with different grid sizes
• ✅ Seed parsing with valid/invalid formats
• ✅ Seed auto-fixing for too short/long sequences
• ✅ Flood fill algorithm with various scenarios:
  • Simple 2x2 grids
  • Connected vs disconnected regions
  • Edge cases (out of bounds, empty grids)
  • Target equals replacement color

src/main.test.tsx (2 tests)

• ✅ Module import validation
• ✅ Mock dependency verification

src/app.test.tsx (6 tests)

• ✅ Component rendering (title, buttons, moves counter)
• ✅ Game state display (seed, grid cells)
• ✅ Initial button states (restart disabled)

🛠️ Updated Scripts

• Updated package.json test script from placeholder to vitest run
• All existing build and lint processes remain functional

Verification

npm test        # ✅ 23 tests pass
npm run build   # ✅ Builds successfully  
npm run lint    # ✅ Passes TypeScript and oxlint checks

The implementation provides a solid foundation for testing game logic and component behavior while maintaining the existing functionality unchanged.

Fixes #501.

---

💬 Share your feedback on Copilot coding agent for the chance to win a $200 gift card! Click here to start the survey.

[Shuunen]

please ignore this dep "@vitest/coverage-v8" in knip config

[Copilot]

please ignore this dep "@vitest/coverage-v8" in knip config

Added @vitest/coverage-v8 to knip ignored dependencies in a2d4cd6