Fix error message for duplicate firm invite

Improve the error message shown when a player sends a duplicate firm employment invite.

Grounded — the message originates in FirmRequestServiceImpl.offerEmployment (business-rian/.../services/impl/FirmRequestServiceImpl.java:64-65): hasPendingJobOffer(...) → BadCommandException("This user already has a pending invite to join this firm."), surfaced via RequestCommands.offer. (Screenshot in the original report.)

Make it clearer / more actionable — e.g. name the player + firm and suggest the next step (await acceptance or revoke the existing invite) — routed through the plugin's message keys rather than a hard-coded string (ties into PAR-16 if that lands).

Note: re-filed from the Tesks project to Business — this is a business-rian firm-invite concern, not a Tesks-app issue.