Build multi-signature co-signer approval UI with invitation flow and status tracking

[Kingsman-99]

Overview

When an invoice requires N-of-M co-signer approvals before release, the UI must let the creator invite co-signers, track who has approved, and let co-signers approve from a dedicated page.

Requirements

• Extend /invoice/new form with optional "Require co-signer approvals" section: add co-signer addresses, set threshold
• /invoice/[id]/cosigners page: shows list of required co-signers with approval status (pending / approved / threshold met)
• Co-signer can approve from this page using their connected Freighter wallet
• Real-time status updates via useInvoiceStream
• Email/address invitation copy button for sharing the co-signer link
• Mobile-responsive layout
• Write unit tests for the co-signer status component

Acceptance Criteria

• Invoice creation with co-signers works end-to-end
• Co-signer status page shows correct approval states
• Approve button works for connected co-signer wallets
• Real-time updates via event stream
• Mobile-responsive on all screen sizes
• npm run lint passes with zero errors
• TypeScript compiles with zero errors
• npm run build succeeds

Definition of Done

All CI checks must pass before the PR is reviewed.

[nurudeenheroic]

Hi! I'd love to work on this. I'm confident I can deliver a clean solution.

[grantfox-oss]

🦊 GrantFox — @nurudeenheroic has been assigned to this issue as part of the Official Campaign | FWC26 campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #383)
2. Your PR will be reviewed by the Stellar-split maintainers

Good luck! Track your progress on GrantFox.