Update paddle collision angle semantics

We should either randomize the angle or make it dependent e.g. on how far from the center the ball hits to make the game a bit more interesting.

Start by locating the paddle collision handling and determine how the current ball angle is selected. Decide whether the angle should be randomized or based on the ball's distance from the paddle center, then verify through gameplay that collisions produce varied, playable trajectories.

Written by the indexing model from the issue text.