feat(payment): add chat support discounts and gratitude

(cherry picked from commit ef9b79bc83)

src/stores/chat/__tests__/chat-commercial-message.test.ts

@@ -0,0 +1,34 @@
+import { describe, expect, it } from "vitest";
+
+import { appendCommercialMessage } from "../helper/commercial-message";
+
+describe("chat commercial message", () => {
+  const incoming = {
+    messageId: "payment-thanks-1",
+    message: "Thank you for supporting me.",
+    characterId: "maya-tan",
+    createdAt: "2026-07-28T08:30:00.000Z",
+  };
+
+  it("appends one persisted payment thank-you to the matching character chat", () => {
+    const messages = appendCommercialMessage([], "maya-tan", incoming);
+
+    expect(messages).toHaveLength(1);
+    expect(messages[0]).toMatchObject({
+      remoteId: "payment-thanks-1",
+      content: "Thank you for supporting me.",
+      isFromAI: true,
+    });
+  });
+
+  it("ignores duplicate delivery and a message for another character", () => {
+    const messages = appendCommercialMessage([], "maya-tan", incoming);
+
+    expect(appendCommercialMessage(messages, "maya-tan", incoming)).toEqual(
+      messages,
+    );
+    expect(appendCommercialMessage(messages, "elio", incoming)).toEqual(
+      messages,
+    );
+  });
+});