Item Purge

### v3.0.6 — Config Fix

**Fixed**
- Fixed the purge interval being ignored on startup and reverting to default
  after every restart. Settings now persist properly whether you change them
  with commands or by editing the config file.
- Fixed in-game command changes (interval, warning, broadcast, whitelist)
  not being written to the config file.
- Fixed config reload events not being detected.

**Changed**
- The config file moved from the per-world
  `<world>/serverconfig/itempurge-server.toml` to a single global
  `config/itempurge-common.toml`. One file now applies to every world instead
  of each world keeping its own copy.
- Any existing `config/itempurge.json` is imported automatically on first
  launch, then renamed to `itempurge.json.imported`. No settings are lost and
  no manual steps are needed.
- The purge now follows each dimension's actual build height instead of a
  fixed -64 to 320 range, so modded dimensions with taller or deeper worlds
  are fully covered.

**Note for server admins:** if you previously customised
`serverconfig/itempurge-server.toml`, that file is no longer read. Your
settings carry over automatically from the JSON config; anything else should
be re-applied in `config/itempurge-common.toml`.