Wire "I paid" chat action to `POST /escrow/single-release/v2/change-milestone-status`

[grantfox-oss]

Context

When the buyer sends fiat outside the platform (SEPA/Bizum), they need to mark the milestone as completed from the P2P chat.

Task

In /p2p/chat, when the buyer confirms payment (existing PaymentBubble "Pagar" action), call POST /escrow/single-release/v2/change-milestone-status with { contractId, serviceProvider: buyerAddress, updates: [{ index: 0, newStatus: "completed" }] }, then sign and submit the returned unsignedXdr.

Acceptance criteria

• Buyer's wallet is prompted to sign the resulting XDR.
• Chat UI updates the PaymentBubble state (request → sent) only after send-transaction confirms success.
• Seller is notified that the milestone was marked complete and needs approval.

[SudiptaPaul-31]

May I be assigned to this issue please my name is Sudipta Paul a active open source contributor with more than 50+ merged pr's and also an active contributor of GrantFox from begining and would love to work on this issue and get a pr merged

[grantfox-oss]

🦊 GrantFox — @SudiptaPaul-31 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 #309)
2. Your PR will be reviewed by the Kaizenode maintainers

Good luck! Track your progress on GrantFox.

[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Official Campaign | FWC26 campaign!

@SudiptaPaul-31's PR #329 was approved and merged by @diegoTech14.

🏆 @SudiptaPaul-31: You earned 40 FoxPoints for this contribution! Your current tier: Builder (1,484 total points). Track your full progress on GrantFox.

👏 Great work, @SudiptaPaul-31! Keep contributing to Kaizenode.