feat(FR-3243): add SFTP resource group settings from the Resource Group page

Resolves #8113 (FR-3243)

Superadmins can now designate which storage proxy handles SFTP sessions for
one or more resource groups (scaling groups), from the Resource Group list:
- Multi-select resource-group rows via checkboxes -> a 'N selected' label +
  a gear (Settings) button appears (mirrors ProjectStoragePermissionTable).
- The gear opens a modal listing the selected groups and a storage-proxy
  select (first proxy auto-selected).
- On submit, the selection is UNIONED into the chosen proxy's existing etcd
  list at volumes/proxies/{proxy}/sftp_scaling_groups (add-only; etcd set
  overwrites, so the current value is read and merged rather than replaced).

Persisted via a raw signed request to /config/get + /config/set (superadmin
REST), since there is no GraphQL mutation for this etcd key. Also adds a
reusable BAIStorageProxySelect component (+ Storybook) in backend.ai-ui.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: agatha197