Applied comprehensive refactoring based on wscats-projects-refactor-spec.md:

- .editorconfig
- .gitignore
- .prettierrc
- tsconfig.json
- jest.config.js
- .github/workflows/ci.yml
- .github/dependabot.yml
- src/utils/error-handling.ts
- src/utils/security.ts
- src/utils/performance.ts
- src/utils/index.ts
- src/utils/__tests__/error-handling.test.ts
- src/utils/__tests__/security.test.ts
- src/utils/__tests__/performance.test.ts
- src/refactored/piano-player.ts
- src/refactored/piano-player.test.ts
- package.json
- REFACTOR.md

Key additions:
- TypeScript utility modules (error-handling, security, performance)
- Comprehensive test suite with 80% coverage threshold
- Project-specific code refactoring
- ESLint + Prettier + TypeScript strict mode
- GitHub Actions CI/CD pipeline

Changes:
- .editorconfig
- .gitignore
- .prettierrc
- .github/workflows/ci.yml
- .github/dependabot.yml
- LICENSE
- CONTRIBUTING.md
- removed .eslintrc
- .eslintrc.json
- tsconfig.json
- jest.config.js
- src/utils/security.ts
- src/utils/error-handling.ts
- src/utils/performance.ts
- src/utils/index.ts
- src/utils/__tests__/security.test.ts
- src/utils/__tests__/error-handling.test.ts
- src/utils/__tests__/performance.test.ts
- 10 JS files modernized
- package.json
- REFACTOR.md

Key improvements:
- Modernized JS: var→const/let, ==→===, 'use strict'
- Stricter ESLint & TypeScript configs
- Added security, error-handling, performance utilities
- 80% coverage threshold, Node 18/20/22 CI matrix
- Added LICENSE, CONTRIBUTING.md

- Remove ~5 console.log from playNote, stopSong, playSong, handleString
- Fix deprecated arguments.callee.caller usage
- Add null check for missing notes in playNote
- Remove dead createPianoKeys() function (~80 lines)
- Remove commented-out CSS (user-select, transition, font-size)
- Replace Chinese CSS comments with English

- Convert all 15 JS source files to TypeScript
- Create types.ts with full type definitions (NotesMap, PianoKeyPair, Song, etc.)
- Create omi.d.ts with Omi framework type declarations
- Add PianoData, FooterData interfaces for component data
- Type PianoStore interface for state management
- Extract KEY_CODE_TO_NOTE/KEY_CODE_TO_SHARP maps (replace 2 switch blocks)
- Extract gradient constants (WHITE_KEY_BG, ACTIVE_KEY_BG, BLACK_KEY_BG)
- Extract timing constants (KEY_HIGHLIGHT_DURATION, NOTE_COOLDOWN_DURATION)
- Add JSDoc documentation to all public methods
- Fix malformed test.ts (loose expressions → proper typed array)
- Replace e.returnValue with e.preventDefault()
- Remove vendor-prefixed CSS duplicates
- Update tsconfig: allowJs=false, add jsx/jsxFactory config
- Net reduction: 291 lines