More reliable SkyWars kit menus

SkyWars progression now initializes safely when several server tasks load a player's kit data at the same time.

• Opening the SkyWars kit menu no longer fails when progression data is being created concurrently.
• The fix applies to both Java and Bedrock players without changing existing kits, coins, XP or selections.
• Existing progression remains untouched; only first-time initialization is serialized.