Fix initial saturation for new challenge joins

Set player saturation to 6f instead of inheriting lobby's 20f when first joining a challenge. Reconnecting players still get their saved saturation from database.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: finreinhard

src/main/kotlin/li/angu/challengeplugin/models/Challenge.kt

@@ -162,6 +162,7 @@ class Challenge(
         player.gameMode = GameMode.SURVIVAL
         player.health = 20.0
         player.foodLevel = 20
+        player.saturation = 6f
         player.exp = 0f
         player.level = 0
         player.inventory.clear()