Implement XP and leveling system and refactor loot dropping
- Add XP field to items and implement XP gain and leveling in Player.
- Introduce LvlUpUi for visual feedback on level up.
- Refactor loot dropping logic into Mob::dropLoot base method.
- Update Entropy, Essence, Harmony, and weapons with XP values.
- Add level-up SFX and Bitwig project assets.
- Fix JSON parsing in Respawner and update journal with progress.