Feat/refund management UI

[Ekene001]

closes #89

Description:

Adds a complete refund management system to the payment dashboard, including a refund action flow in the payment detail drawer, a dedicated refunds page, and real-time webhook-driven UI updates.

What this PR does

Refund action in payment detail

• "Issue Refund" button in [PaymentDetailDrawer] visible only when the payment is completed, within the 180-day refund window, and not already fully refunded
• Refund status badge (Refund Pending / Refunded / Partial Refund) displayed alongside primary status
• Refund detail cards showing amount, status, estimated delivery, and notes
• Refund activity appended to the payment timeline

Refund modal

• Two-step flow: form → confirmation
• Full/partial refund toggle with segmented control
• Amount field: pre-filled, readonly for full refunds, validated for partial (no zero/negative, cannot exceed original)
• Required reason dropdown (customer_request, duplicate, fraudulent, other)
• Optional notes textarea with 500-char limit and live counter
• Refund rules info block with days remaining and estimated delivery
• Confirmation step with plain-language warning about irreversibility and processing time
• Loading state and success/error toast notifications on submission

Refund history page

• New [/refunds] route with filterable, sortable, paginated table
• Columns: Payment ID, Refund Amount, Reason, Status, Initiated Date, Completed Date
• Status filter tabs: All, Pending, Processing, Completed, Failed
• Added "Refunds" nav item to sidebar

Webhook event support

• [useRefundEvents] hook subscribes to [refund:created] and [refund:completed] socket events
• Auto-invalidates payment, refund, and list caches on event receipt
• Toast notifications for real-time status changes
• Integrated at dashboard layout level so it runs across all pages

Status badge updates

• [REFUNDING]→ "Refund Pending" (yellow/pending variant)
• [REFUNDED] → "Refunded" (green/completed variant)
• New [RefundStatusBadge] component for Partial Refund (blue/processing variant)
• [PaymentTimeline] now renders REFUNDED/REFUNDING as completed payment flows instead of error states

Helpers

• [getRefundEta(paymentMethod)] — returns estimated delivery time (Card: 5–10 days, Bank: 1–3 days, Crypto: minutes)
• [canRefund()], isWithinRefundWindow(), [refundWindowDaysRemaining()]
• Refund types, payload interfaces, and reason constants

New files

• src/lib/refund.ts — types, constants, helper functions
• src/hooks/useRefunds.ts — React Query hooks for refund CRUD
• src/hooks/useRefundEvents.ts — WebSocket event listener
• src/components/payments/RefundModal.tsx — 2-step refund dialog
• src/components/payments/RefundTimeline.tsx — refund activity timeline
• src/components/payments/RefundStatusBadge.tsx — refund state badge
• src/components/payments/RefundHistoryTable.tsx — filterable refund table
• src/app/(dashboard)/refunds/page.tsx — refunds page route

Modified files

• src/components/payments/PaymentDetailDrawer.tsx — refund button, badges, detail section, timeline
• src/components/payments/StatusBadge.tsx — updated variants/labels for refund states
• src/components/payments/PaymentTimeline.tsx — refund-aware timeline rendering
• src/components/app-sidebar.tsx — added Refunds nav item
• src/app/(dashboard)/layout.tsx — wired up [useRefundEvents()]

[Ekene001]

@0xdevcollins pls review