[FE-49] Build multi-signature transfer approval UI — pending approvals queue and sign flow

[yusuftomilola]

Overview

Some asset transfers require approval from multiple authorised signers before they execute — enforced by the multisig_transfer smart contract. This UI shows pending transfer requests and allows signers to review and approve or reject them. Depends on BE-61 and the multisig contract.

Context

• Route: frontend/app/(dashboard)/approvals/page.tsx
• The multisig_transfer contract (contracts/multisig_transfer/) manages approval rules and transfer requests
• Backend will need a GET /stellar/transfers/pending and POST /stellar/transfers/:id/approve endpoint (coordinate with BE-61)

Acceptance Criteria

• Create /approvals page showing all pending multi-sig transfer requests for the current user
• Each request shows: asset name and ID, transfer from, transfer to, requested by, required signatures, current signatures, expiry date
• Progress bar: "2 of 3 required approvals collected"
• "Approve" button: opens a confirmation dialog with full transfer details before calling the approve endpoint
• "Reject" button: opens a dialog to enter rejection reason
• Approved/rejected items archived in a "Completed" tab with the outcome and final transaction hash
• Badge on the sidebar notification for pending approvals count