[upstream #4594] feat(core, dashboard): Support pluggable refund destinations

[ayim]

Mirrored from vendurehq#4594 for the Overwatch review demo.

Original author: @oliverstreissi

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Description

Add RefundDestinationStrategy interface allowing plugins to define alternative refund destinations (store credit, gift cards, etc.) instead of only refunding to the original payment method.

Backend:

• New RefundDestinationStrategy interface with isAvailable/createRefund methods
• refundDestinations query to discover available destinations per order
• destination field on RefundOrderInput and Refund entity
• Routes PaymentService.createRefund through destination strategy when specified
• Startup validation rejecting strategies using reserved default code
• Uses refundable payment capacity (not just Settled state) for availability checks

Dashboard:

• Refund dialog split into "Payment methods" and "Other refund destinations" sections
• Independent allocation with payments prioritized before destinations
• Fix multi-target refund sending duplicate RefundLine records
• Remove orphaned refundFragment and dead utility functions

Fixes vendurehq#4563

Breaking changes

None. Omitting destination in RefundOrderInput preserves existing behavior.

Screenshots

Screen.Recording.2026-03-31.at.10.21.26.mov

Checklist

📌 Always:

• I have set a clear title
• My PR is small and contains a single feature
• I have checked my own PR

👍 Most of the time:

• I have added or updated test cases
• I have updated the README if needed