feat(chat): add quota-exhausted banner for guest users

Introduce ChatQuotaExhaustedBanner component that displays a pink gradient
banner with an "Unlock your membership to continue" CTA when a guest user's
free chat quota has been exhausted. The banner is shown in ChatScreen when
isGuest is true, quota has loaded, and the chat state machine's
quotaExceededTrigger has been incremented by a quota guard. Default click
navigates to /subscription, with an optional onUnlock callback for
overrides/tests. Styles align with the existing splash and sidebar VIP
card visual language.

src/stores/chat/chat-machine.ts

@@ -33,7 +33,8 @@
 import { setup, assign } from "xstate";
 
 import { ChatStorage } from "@/data/storage/chat/chat_storage";
-import { formatDate } from "@/utils/date";
+import { formatDate, todayString } from "@/utils/date";
+
 
 import { ChatState, initialState } from "./chat-state";
 import type { ChatEvent } from "./chat-events";
@@ -85,8 +86,9 @@ export const chatMachine = setup({
 
       // 持久化两个额度（ fire-and-forget，不 await —— assign 是 sync）
       // daily quota 需要 today 字符串（让 ChatStorage 跨天判断 reset）
-      const today = formatDate();
+      const today = todayString();
       ChatStorage.getInstance().setGuestDailyChatQuota(newRemaining, today);
+
       ChatStorage.getInstance().setGuestTotalQuota(newTotal);
 
       console.log("[chat-machine] appendUserMessage", {
@@ -124,9 +126,12 @@ export const chatMachine = setup({
       const newTotal = context.wsConnected
         ? context.guestTotalQuota
         : Math.max(0, context.guestTotalQuota - 1);
-      const today = formatDate();
+        
+      const today = todayString();
       void ChatStorage.getInstance().setGuestDailyChatQuota(newRemaining, today);
+
       void ChatStorage.getInstance().setGuestTotalQuota(newTotal);
+      
       console.log("[chat-machine] appendUserImage", {
         oldMessagesCount: context.messages.length,
         wsConnected: context.wsConnected,