Add background switching functionality with UI selector

[Null000]

Summary

This PR adds the ability to dynamically switch between different background types during gameplay through a new UI control panel.

Key Changes

• New switchBackground() function: Manages background transitions by removing the current background and adding a new one at the correct z-index (right after the mask layer)
• Background state tracking: Added currentBackground variable to keep track of the active background sprite
• UI control panel: Added a dropdown selector in the HTML with 8 background options (Village, Forest, Dark Forest, Dungeon, Swamp, Mountains, Graveyard, Volcano)
• Event listener: Wired up the background selector to trigger background switches and log the change
• Refactored initialization: Updated the init() function to use the new switchBackground() function instead of directly creating and adding the background

Implementation Details

• The switchBackground() function properly cleans up the previous background before adding a new one
• Backgrounds are inserted at index 1 in the world container to maintain proper layering (after the mask)
• The background selector uses the BackgroundType enum values for consistency
• User actions are logged to the console for debugging purposes

https://claude.ai/code/session_01MS7EGJedRFXeSceQaiznNW