Add playable browser-based Defender game

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Motivation

• Provide a small, dependency-free browser game to serve as a demo/feature: a simple Defender-style shooter with scoring and lives.
• Offer an interactive example that showcases canvas rendering, game loop, and input handling for use in demos or local hosting.

Description

• Add index.html which mounts the canvas, HUD (score/lives), help text, and a Restart button wired to the game.
• Add style.css that provides a dark-space visual style, responsive canvas sizing, and HUD/button presentation.
• Add game.js implementing the game loop, player movement and rendering, shooting with cooldown, enemy spawning with difficulty scaling, bullet/enemy collision detection, scoring, lives, and a game-over overlay with restart support.
• The game is dependency-free and runs by opening index.html in a browser or serving the directory with a static server.

Testing

• Ran node --check game.js which completed successfully.

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Codex Task