Fixed inconsistent game speed + restored CRT terminal UI

My game was running at different speeds depending on the monitor refresh rate. I fixed the game loop to use a fixed 240Hz timestep so it feels identical everywhere.

Also switched the entire UI to the CRT terminal theme: green phosphor text (#33ff00), VT323 monospace font, CRT scanline overlay, screen flicker effect, and chunky retro buttons. All inline styles moved to an external CSS file.

Files changed: game.js, index.html, style.css