fix (notifications): prevent hydration mismatch in notifications trigger

[dhaanisi]

Summary

This PR prevents a hydration mismatch in the notifications trigger component.

NotificationIconButton from @knocklabs/react can render different markup between the server and client (for example, theme-dependent classes like light/dark variants). When rendered during SSR, this can cause a React hydration warning in development.

Changes

• Added a client mount guard using mounted state with useEffect to ensure the component renders only after the client has mounted.
• Simplified render guards and avoided repeated keys() calls.

if (!mounted || !NEXT_PUBLIC_KNOCK_API_KEY) {
  return null;
}