Server owners can now protect either a spawn radius or an entire lobby/hub world, set a custom protection center in-game, and preview the protected radius with particles.

### Added
- Added `protection-area.mode`.
- `radius` protects an area around the configured center.
- `world` protects the entire configured world.
- Added full world protection mode.
- Ideal for dedicated lobby or hub worlds.
- No WorldGuard regions required.
- No complex setup required.
- Added custom protection center support.
- Use `/spawnprotection setcenter` to set the protection center at your current location.
- Use `/spawnprotection resetcenter` to return to the configured world spawn.
- Added radius preview.
- Use `/spawnprotection preview` to display the protected radius with particles.
- Helps server owners clearly see where protection starts and ends.
- Added new config options:
```yaml
protection-area:
mode: "radius"
spawn:
use-custom-center: false
center:
x: 0.5
y: 64.0
z: 0.5