Add direct transfers between a firm's accounts in-game

Players currently have to withdraw to their personal balance and deposit again to move funds between accounts belonging to the same firm.

Add /business account transfer <firm> <fromAccountId> <toAccountId> <amount> [memo] to move funds directly through the Treasury ledger.

Acceptance criteria:

• Both accounts must be live accounts linked to the specified firm, including subaccounts.
• Require the existing firm finance permission and spending authority on the source account.
• Reject identical accounts, invalid amounts, insufficient funds, and accounts outside the firm without moving money.
• Perform one atomic ledger transfer, preserving an optional memo and existing audit behavior; do not involve a personal account.
• Provide player-facing success/error messages and command documentation.
• Add regression coverage for balances, authorization, validation, and memo recording.