Feature: Add reverse thrust (down arrow key)

[leighmcculloch]

Description

Add reverse thrust capability mapped to the down arrow key.

Motivation

• When instinctively hitting the down arrow key, the browser scrolls down instead of the ship decelerating or reversing — this breaks immersion.
• Reverse thrust would also just be a cool addition to ship handling.

Suggested behavior

• Down arrow key applies thrust in the opposite direction the ship is facing (or opposite to current velocity).
• Prevents the default browser scroll behavior when the game is active.