fix(chat): stabilize message identities

src/stores/chat/__tests__/chat-promotion.test.ts

@@ -22,7 +22,7 @@ describe("chat promotion", () => {
     const messages = appendPromotionMessage(
       [
         {
-          id: "history-1",
+          displayId: "history-1",
           content: "History",
           isFromAI: true,
           date: "2026-07-13",
@@ -31,7 +31,7 @@ describe("chat promotion", () => {
       state,
     );
 
-    expect(messages.map((message) => message.id)).toEqual([
+    expect(messages.map((message) => message.displayId)).toEqual([
       "history-1",
       "promotion:promotion-1",
     ]);
@@ -62,7 +62,8 @@ describe("chat promotion", () => {
     });
 
     expect(next?.message).toMatchObject({
-      id: "backend-1",
+      displayId: "promotion:promotion-1",
+      remoteId: "backend-1",
       imageUrl: "https://example.com/unlocked.jpg",
       imagePaywalled: false,
       locked: false,
@@ -71,10 +72,19 @@ describe("chat promotion", () => {
 
   it("keeps the promotion last and removes matching history duplicates", () => {
     const state = createChatPromotionState(promotion, "backend-1");
+    const userMessage = {
+      displayId: "server:backend-1:user",
+      remoteId: "backend-1",
+      content: "User message with the shared remote id",
+      isFromAI: false,
+      date: "2026-07-13",
+    };
     const messages = appendPromotionMessage(
       [
+        userMessage,
         {
-          id: "backend-1",
+          displayId: "server:backend-1:assistant",
+          remoteId: "backend-1",
           content: "Stale history copy",
           isFromAI: true,
           date: "2026-07-13",
@@ -83,6 +93,6 @@ describe("chat promotion", () => {
       state,
     );
 
-    expect(messages).toEqual([state.message]);
+    expect(messages).toEqual([userMessage, state.message]);
   });
 });