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.

components/header.tsx

@@ -34,7 +34,7 @@ import {
 } from "lucide-react"
 import Link from "next/link"
 import { usePathname, useRouter } from "next/navigation"
-import { useState } from "react"
+import { useEffect, useState } from "react"
 
 import { canAccessDashboard } from "@/components/role-guard"
 
@@ -59,6 +59,12 @@ export function Header() {
   } = useAuthStore()
   const canOpenDashboard = currentRole === "player" || currentRole === "owner"
   const { shop: ownerShop } = useMyShop(isAuthenticated && currentRole === "owner")
+  const fetchNotifications = useNotificationStore((state) => state.fetchNotifications)
+
+  useEffect(() => {
+    if (!isAuthenticated) return
+    void fetchNotifications()
+  }, [fetchNotifications, isAuthenticated])
 
   const navLinks =
     currentRole === "consumer"
@@ -110,6 +116,7 @@ export function Header() {
         notifyInfo(toApiError(error).msg)
       } finally {
         clearAuth()
+        useNotificationStore.getState().invalidate()
         setMobileOpen(false)
         router.push("/")
       }