Boss Arena

Added per-arena notification radius

• New notificationRadius field on arenas (stored in arenas.json).

• Arenas tab now shows a Notify column; each arena row has an editable notification radius in blocks.

• Boss event titles now use the arena’s radius; falls back to config.json.notificationRadius when not set.

Timed spawns respect player presence

• Timed boss spawns now only fire when at least one player is online in the target world.

• If a spawn time occurs while no players are online, the rule is deferred and retried every 30 seconds until a player joins, then a single boss is spawned.

• Existing options preventDuplicateWhileAlive and despawnAfterHours/Minutes continue to work as before, preventing stacking and cleaning up old events.