Introduces dynamic background images for `BattleScene` based on the opponent's faction (Imperial, Raider, Bloodthirsty). The system:
- Defines a configuration of 3 variant assets per faction.
- Selects a random variant during the preload phase if the opponent's commander card is available.
- Renders the selected background with a dimming overlay in `create`, falling back to the solid color if no image is loaded or defined.
Includes new asset files: `imperial_03.png` and `raider_03.png`.
