Fix floating-point display imprecision for cost/increment values in admin panels

[Copilot]

Cost values entered by admins (e.g. 0.03) were displayed as their full IEEE 754 double representation (0.0299999999329447746) in GUI tooltips due to bare String.valueOf(double) calls.

Changes

• Utils — Added formatDouble(double) using a ThreadLocal<DecimalFormat> with pattern "0.###": up to 3 decimal places, trailing zeros stripped, thread-safe for concurrent panel access across players.

// Before
String.valueOf(this.biome.getCostIncrement())  // → "0.0299999999329447746"

// After
Utils.formatDouble(this.biome.getCostIncrement())  // → "0.03"

• BiomeEditPanel — Applied Utils.formatDouble() to the three double-typed cost fields displayed in the admin edit panel: unlock cost, change cost, and cost increment.
• CommonPanel — Applied Utils.formatDouble() to unlock cost in the biome description lore.