Randomize initial serve direction in Pong game

[Copilot]

The ball always started moving right (towards AI), giving the human player first serve every time.

Changes

• Randomized serve initialization: 50/50 chance for either player to serve first
• Aligned ball velocity: Initial velocity now matches randomly selected serve direction

// Before: always served right
this.serve = 1;
this.ball = new Ball(
  new Vector2(this.width / 2, this.height / 2),
  new Vector2(5, 0),  // always positive X
  ...
);

// After: random serve direction
this.serve = Math.random() < 0.5 ? 1 : -1;
this.ball = new Ball(
  new Vector2(this.width / 2, this.height / 2),
  new Vector2(5 * this.serve, 0),  // matches serve direction
  ...
);

The existing serve alternation logic (paddle hits, post-score resets) remains unchanged.

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