Feat/tip refund mechanism

[Mosas2000]

Smart contract (
tipstream.clar
)

Added 5 new error constants (err-refund-window-expired, err-already-refunded, err-not-tip-sender, err-refund-not-found, err-refund-not-pending)
Added refund-window-blocks constant (144 blocks ≈ 24 hours) and status constants
Added refund-requests and refunded-tips data maps
Added request-refund, approve-refund, reject-refund public functions
Added get-refund-request, is-tip-refunded, is-refund-eligible, get-refund-window-blocks read-only functions
Backend (chainhook/)

storage.js — MemoryRefundStore and PostgresRefundStore with full CRUD, createRefundStore factory, REFUND_STATUSES and REFUND_WINDOW_MS exports
schema.sql — refund_requests table with indexes on sender, recipient, status
server.js — POST /api/refunds, GET /api/refunds, GET /api/refunds/:tipId, PATCH /api/refunds/:tipId endpoints with full validation
refund.test.js — 16 store unit tests
refund-api.test.js — 18 validation and lifecycle tests
Frontend (frontend/src/)

contracts.js
— 7 new refund function name constants
routes.js
— ROUTE_REFUNDS with labels, titles, and metadata
useRefund.js
— useRefund hook with requestRefund, resolveRefund, fetchRefundRequest, fetchUserRefunds, isWithinRefundWindow
RefundRequest.jsx
— sender-side refund initiation with confirm dialog and on-chain call
RefundApproval.jsx
— recipient-side approve/reject with on-chain calls
RefundHistory.jsx
— paginated history with direction and status filters
RefundManager.jsx
— page-level container
TipHistory.jsx
— integrated RefundRequest and RefundApproval into each tip row
App.jsx — ROUTE_REFUNDS route, RefundManager lazy import, RotateCcw nav icon, addToast passed to TipHistory
RefundRequest.test.jsx — 12 component tests
RefundHistory.test.jsx — 12 component tests

Closes #392