refactor(notifications): fetch from backend API instead of local generation

Rewrite store/notifications.ts to fetch via listNotifications
API and remove local generateId-based notification creation.
The store now acts as a simple cache with fetch/invalidate methods.
Header unread count reads from this API-backed cache.

store/notifications.ts

@@ -1,48 +1,36 @@
-import { generateId } from "@/lib/id"
+import { listNotifications } from "@/lib/api/notifications"
 import type { Notification } from "@/lib/types"
 import { create } from "zustand"
 
-interface CreateNotificationInput {
-  type: Notification["type"]
-  title: string
-  content: string
-  link?: string
-}
-
 interface NotificationState {
   notifications: Notification[]
-  addNotification: (input: CreateNotificationInput) => Notification
-  markAsRead: (notificationId: string) => void
-  markAllAsRead: () => void
+  loading: boolean
+  fetchNotifications: () => Promise<void>
+  setNotifications: (notifications: Notification[]) => void
+  markAllRead: () => void
+  invalidate: () => void
 }
 
 export const useNotificationStore = create<NotificationState>((set) => ({
   notifications: [],
-  addNotification: (input) => {
-    const notification: Notification = {
-      id: generateId("notif"),
-      type: input.type,
-      title: input.title,
-      content: input.content,
-      link: input.link,
-      read: false,
-      createdAt: new Date().toISOString(),
+  loading: false,
+  fetchNotifications: async () => {
+    set({ loading: true })
+    try {
+      const items = await listNotifications({ offset: 0, limit: 50 })
+      set({ notifications: items, loading: false })
+    } catch {
+      set({ loading: false })
     }
-
-    set((state) => ({
-      notifications: [notification, ...state.notifications],
-    }))
-
-    return notification
   },
-  markAsRead: (notificationId) =>
+  setNotifications: (notifications) => set({ notifications }),
+  markAllRead: () =>
     set((state) => ({
       notifications: state.notifications.map((notification) =>
-        notification.id === notificationId ? { ...notification, read: true } : notification,
+        notification.read ? notification : { ...notification, read: true },
       ),
     })),
-  markAllAsRead: () =>
-    set((state) => ({
-      notifications: state.notifications.map((notification) => ({ ...notification, read: true })),
-    })),
+  invalidate: () => {
+    set({ notifications: [] })
+  },
 }))