B2B Transfers Enhancement

Goal

Make firm-to-firm (B2B) money movement easier: let a player who controls multiple firms transfer between them smoothly, with a menu to drive it.

Today (grounded)

• Paying into a firm exists: MiscCommands.payInto (pay into <firm> <amount>, open) and a member-gated deposit <firm> <amount>. A player's firms: FirmApi.getPlayerFirms(uuid). Firm balance: FirmApi.getTotalBalance (PAR-22).
• There is no first-class "move money from firm A to firm B" flow, and no menu — it's manual pay-ins.

To design / build

• A B2B transfer path (firm→firm) routed through TreasuryApi.transfer (ledger stays authoritative — do not add a side balance store), gated on the sender holding FINANCIAL/ADMIN on the source firm.
• A GUI/menu: pick source firm → target firm → amount, listing the player's firms + balances.
• Tag the transfer with a plugin_system (so it's attributable — cf. PAR-145).

Open questions (decide before building)

• What "accounts sync with multiple companies" means precisely — a saved set of linked firms? a quick-transfer favourites list? a shared authorizer?
• Permission model for moving firm money out (FINANCIAL vs ADMIN).
• Are non-controlled target firms allowed (if so it's just pay into)?

Acceptance

• A multi-firm operator can transfer between their own firms in a few clicks; every movement is a normal, attributed ledger transfer.

(High priority but was under-specified — this adds structure; the owner/agent should resolve the open questions first.)