refactor(FR-2494): improve auto scaling rule v2 UI — loading, unmount, step size sign addon, threshold % suffix (#6749)

Resolves #6749 (FR-2494)

## Changes

- **Loading indicator**: Refresh button shows loading state on both initial Prometheus preset selection and manual refresh
- **BAIUnmountAfterClose**: Wrap editor modal to properly unmount after close
- **Step size sign addon**: Use `Space.Compact` + `Space.Addon` to display +/−/± as a prefix (range: ±, scale-out: +, scale-in: −)
- **Threshold % suffix**: Show % unit suffix on threshold inputs when metric source is KERNEL
- **Suspense boundary**: Add Suspense boundary inside the modal to prevent the rule list from refreshing when the modal opens
- **Skeleton fallback**: Show `Skeleton` instead of `Spin` while the modal content loads

Author: agatha197