feat: Announcement Popup Notification (#411)

* docs: add announcement popup design spec

* docs: add announcement popup implementation plan

* feat(api): include update_time in announcement response

* feat(ui): add update_time to AnnouncementData interface

* feat(ui): create AnnouncementModal component

* feat(ui): manage announcement modal state in dashboard hook

* feat(ui): add announcement modal to dashboard layout

vite-frontend/src/pages/dashboard/components/announcement-modal.tsx

@@ -0,0 +1,48 @@
+import type { AnnouncementData } from "@/api";
+import { Button } from "@/shadcn-bridge/heroui/button";
+import {
+  Modal,
+  ModalBody,
+  ModalContent,
+  ModalFooter,
+  ModalHeader,
+} from "@/shadcn-bridge/heroui/modal";
+import ReactMarkdown from "react-markdown";
+import remarkGfm from "remark-gfm";
+
+interface AnnouncementModalProps {
+  announcement: AnnouncementData;
+  isOpen: boolean;
+  onClose: () => void;
+  onDontShowAgain: () => void;
+}
+
+export const AnnouncementModal = ({
+  announcement,
+  isOpen,
+  onClose,
+  onDontShowAgain,
+}: AnnouncementModalProps) => {
+  return (
+    <Modal isOpen={isOpen} onOpenChange={(open) => !open && onClose()} size="2xl">
+      <ModalContent>
+        <ModalHeader className="flex flex-col gap-1">平台公告</ModalHeader>
+        <ModalBody>
+          <div className="prose prose-sm dark:prose-invert max-w-none max-h-[60vh] overflow-y-auto">
+            <ReactMarkdown remarkPlugins={[remarkGfm]}>
+              {announcement.content}
+            </ReactMarkdown>
+          </div>
+        </ModalBody>
+        <ModalFooter>
+          <Button variant="flat" onPress={onDontShowAgain}>
+            不再提示
+          </Button>
+          <Button color="primary" onPress={onClose}>
+            关闭
+          </Button>
+        </ModalFooter>
+      </ModalContent>
+    </Modal>
+  );
+};