feat: replace notification checkboxes with card-based layout (#5421)

Replace EventBusCheckboxes with card-based notification settings:
- Each event group gets its own card with responsive grid layout
- Master checkbox per group with indeterminate state
- Inline parameter inputs (CPU threshold) appear when enabled
- Theme-adaptive via Ant Design Card component

Components:
- NotificationLayout, NotificationCard, NotificationHeader, NotificationEvent
- TelegramNotifications, EmailNotifications with explicit event configs

frontend/src/components/ui/notifications/NotificationLayout.tsx

@@ -0,0 +1,13 @@
+import type { ReactNode } from 'react';
+
+interface Props {
+  children: ReactNode;
+}
+
+export function NotificationLayout({ children }: Props) {
+  return (
+    <div style={{ display: 'grid', gridTemplateColumns: 'repeat(auto-fit, minmax(260px, 1fr))', gap: 12 }}>
+      {children}
+    </div>
+  );
+}