feat(frontend): optimistic UI for accept_offer and repay_invoice

[samjay8]

Problem

Accepting an offer and repaying are slow (wallet sign -> RPC -> confirm) with no interim feedback beyond a spinner. Optimistic updates make the app feel responsive and reduce double-submit confusion.

Scope

• Optimistically update invoice/offer state in the local cache on accept and repay
• Revert on failure with an explanatory toast (wire into the undo-toast work in feat(frontend): undo action on toasts for reversible destructive ops #155)
• Keep a pending indicator so users know the tx is in flight

Stop

• No offline queueing — a failed tx must roll back immediately

Acceptance criteria

• Accepting an offer updates the UI instantly, then reconciles on confirmation
• Failed/declined tx rolls state back cleanly
• Double-submit protection retained

[waterWang]

I'll work on this. accept_offer optimistic UI is already covered by PR #310 (closes #191), which explicitly excluded repayment — so this PR focuses on the remaining gap: optimistic UI for repay_invoice (instant local state update + pending indicator, rollback with an error toast on failure, no offline queueing).