A quality-of-life update focused on safer admin economy actions, better i18n consistency, and faster access to the Business GUI.

## ✨ Improvements
- Prefix precedence in admin messages now respects `config.yml` first, then `messages.yml`, then defaults; reloaded on `/djeconomy reload`.
- Added `/business gui` and `/business menu` helpers to open the business GUI faster, with tab completion. (perm: `djeconomy.gui.access`)

## Bug Fixes
- Offline player deposits now correctly use `EconomyManager.depositPlayer(...)` for offline targets.
- Confirmation threshold/expiry now read with defaults using the correct `FileConfiguration#get*` overloads.
- Added guard for missing economy manager with a friendly `admin.economy_unavailable` message.
- Aligned i18n keys and placeholders across give/take/set, confirm prompts, expiry, offline flow, and reload success.

## Docs
- Updated README/INSTALLATION with JAR guidance and new business GUI commands.

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## Compatibility
- Minecraft: 1.20.4 – 1.21.x
- Integrations: Vault, WorldGuard, McMMO, LuckPerms
- Databases: SQLite, MySQL, MongoDB