refactor(chat): remove private unlock machine flow

src/stores/chat/chat-state.ts

@@ -5,7 +5,7 @@ export interface ChatState {
   isReplyingAI: boolean;
   pendingReplyCount: number;
   upgradePromptVisible: boolean;
-  upgradeReason: "daily_limit" | "private_message" | "image" | null;
+  upgradeReason: "daily_limit" | "image" | null;
   upgradeHint: string | null;
   upgradeDetail:
     | {
@@ -15,7 +15,6 @@ export interface ChatState {
         limit?: number;
       }
     | null;
-  unlockingPrivateMessageId: string | null;
   isLoadingMore: boolean;
   hasMore: boolean;
   historyOffset: number;
@@ -34,7 +33,6 @@ export const initialState: ChatState = {
   upgradeReason: null,
   upgradeHint: null,
   upgradeDetail: null,
-  unlockingPrivateMessageId: null,
   isLoadingMore: false,
   hasMore: true,
   historyOffset: 0,