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/NotificationEvent.tsx

@@ -0,0 +1,26 @@
+import type { ReactNode } from 'react';
+import { Checkbox } from 'antd';
+import { useTranslation } from 'react-i18next';
+
+interface Props {
+  label: string;
+  checked: boolean;
+  onToggle: () => void;
+  children?: ReactNode;
+}
+
+export function NotificationEvent({ label, checked, onToggle, children }: Props) {
+  const { t } = useTranslation();
+  return (
+    <div>
+      <Checkbox checked={checked} onChange={onToggle}>
+        {t(label)}
+      </Checkbox>
+      {checked && children && (
+        <div style={{ paddingLeft: 24, marginTop: 4 }}>
+          {children}
+        </div>
+      )}
+    </div>
+  );
+}