Adds email approval & Postmark integration for negotiations

Enables users to approve and send negotiation emails directly via Postmark after configuring a server token in settings. Introduces new debt statuses ("approved", "sent"), UI for token management, and approval/rejection flows. Refactors notifications to use a modern toast library, adjusts dashboard status filters, and updates DB schema for new flows.

Empowers compliant, user-controlled negotiation and automated email delivery.

src/components/Dashboard.tsx

@@ -169,7 +169,9 @@ export function Dashboard() {
 		active: debts.filter((debt) =>
 			["received", "negotiating"].includes(debt.status)
 		),
-		settled: debts.filter((debt) => debt.status === "settled"),
+		settled: debts.filter((debt) =>
+			["settled", "approved", "sent"].includes(debt.status)
+		),
 		failed: debts.filter((debt) =>
 			["failed", "opted_out"].includes(debt.status)
 		),