Monthly Crates

• Added a server-side monthly crates system with weighted rewards, key tracking, and configurable cooldowns (default 30 days). A starter monthly crate is created automatically if none exist.
• Player commands: /monthlycrate open <crate>, /monthlycrate preview <crate>, /monthlycrate keys [crate], and /monthlycrate list.
• Admin commands: /monthlycrate givekey <player> <crate> [amount], /monthlycrate create <crate> <name> [cooldownDays] [description], /monthlycrate addreward <crate> <reward> <weight> <name> [commands] [message], /monthlycrate removereward <crate> <reward>, /monthlycrate reload.
• Rewards support optional console-run commands (semicolon-separated, underscores become spaces) with placeholders {player}, {player_uuid}, and {reward}; rewards also support a short message shown to the player.
• Keys and last-open timestamps are saved in crates.json inside the mod's data folder; edits there allow deeper customization beyond the commands.
• Limitations: keys can be granted only to online players (command uses live player targets), rewards do not grant physical crate items or GUI spins, and cooldowns are day-based rather than calendar-month aware.