[FEAT] Add Box Game Mode settings

[LeticiaBN]

Is your feature request related to a problem? Please describe.

Box Mode currently has fixed gameplay parameters - only 1 box/hole pair spawns, and the snake speed remains constant throughout the game. Players cannot customize the difficulty or intensity of this game mode, which limits replayability and makes it less engaging for different skill levels.

Describe the solution you'd like

Add customizable settings for Box Mode in the "Game Mode Settings" section:

Setting 1: Number of Boxes

• Name: "Number of boxes"
• Type: Range selector
• Default: 1
• Description: Controls how many box-hole pairs spawn simultaneously on the board
• Behavior: When a box enters its hole, a new box-hole pair spawns to maintain the configured count

Setting 2: Speed Increase on Box Push

• Name: "Speed boost on push"
• Type: Toggle (On/Off)
• Default: Off
• Description: When enabled, the snake's speed increases each time a box is successfully pushed into a hole
• Behavior:
  • Each successful push increases speed by the configured increment (e.g., 5% or 0.5 units)
  • Speed increase follows the same logic as apple eating
  • Respects the max speed limit from settings

Implementation Notes

• Settings should be stored per game mode
• With multiple boxes, prevent them from spawning on top of each other
• Speed increase should use the same system as other speed-increasing mechanics