Add invoice deadline extension request/approval flow

[Kingsman-99]

Description

There's no in-app flow for a recipient to request a deadline extension from the creator, or for the creator to approve/deny it — currently this would require off-platform coordination. This issue builds that flow.

Technical Context

Involves src/app/invoice/[id]/page.tsx. Recipient submits a requested new deadline with a reason; creator sees a pending request banner with approve/deny actions. Approval calls whatever SDK method updates the deadline (or, if no such contract method exists, this issue is scoped to the request/approval UI and local state only, clearly noting the on-chain update is a follow-up).

Acceptance Criteria

• Recipient-only "Request extension" action opens a form for new deadline + reason
• Creator sees a pending-request banner on the invoice with the requested date/reason and approve/deny buttons
• Approve/deny resolves the pending request and notifies the requester of the outcome
• Only one pending request allowed per invoice at a time — new requests blocked while one is pending
• Unit tests: single-pending-request constraint, approve/deny state transitions

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