Add Piano Hero rhythm game with song library and scoring

[mrfeixiang]

Summary

This PR introduces a complete rhythm game implementation called "Piano Hero" with a full-featured web application including gameplay mechanics, audio synthesis, song library, and scoring system.

Key Changes

• Game Engine (GameEngine class): Core game loop with state management (menu, songs, playing, paused, results), song progression, note spawning with lookahead, and hit detection with timing windows
• Audio System (AudioEngine class): Web Audio API integration with synthesized piano sounds using dual oscillators, envelope shaping, and low-pass filtering
• Input Handling (InputManager class): Keyboard and touch/mouse input support for both physical piano keys and on-screen keyboard, with proper event debouncing
• Note Rendering (NoteRenderer class): Falling note animation system with position calculation based on fall speed and hit timing, hit/miss visual feedback
• Scoring System (ScoreTracker class): Hit detection with perfect/good/miss judgments, combo tracking with multipliers, and letter grade calculation
• Song Library (SongLibrary class): Pre-loaded song collection (Twinkle Twinkle, Mary Had a Little Lamb, Hot Cross Buns, Happy Birthday, Ode to Joy) with BPM and difficulty metadata
• UI/UX: Multi-screen interface with main menu, song selection, gameplay HUD, pause overlay, countdown sequence, and results screen with best score tracking
• Styling: Comprehensive CSS with dark theme, gradient effects, responsive layout, color-coded note lanes, and smooth animations

Notable Implementation Details

• Hit detection uses configurable timing windows (PERFECT_WIN: 55ms, GOOD_WIN: 130ms) for skill-based gameplay
• Notes spawn with 2.2-second lookahead to allow smooth visual presentation
• Score multiplier increases with combo (1x → 4x at 30+ combo)
• Piano keyboard visual feedback with color mixing based on note being played
• Local storage integration for tracking best scores per song
• Countdown sequence with visual animation before song starts
• Free Play mode for unrestricted practice without falling notes

https://claude.ai/code/session_011YpsvHY5f8LZtroDhY8Yvn