Disperse Funds & Add Signer Backend (Modals Support)

[HabacucMX]

Implement the backend logic for the Disperse and Add Signer modals:
• Disperse: accept multiple recipient addresses, amounts, and currency; build and submit a Safe transaction (or batch) that sends funds accordingly; return tx status for the modal.
• Add signer: accept new signer address and updated threshold; build and submit the Safe transaction to update the signer set; return tx status and updated signer list.