- Add backend endpoint `/api/s2a/fetch-groups` to fetch S2A groups using arbitrary credentials without depending on global config
- Implement `handleFetchGroups` handler with proper error handling and multiple auth header support (Bearer token, X-API-Key, X-Admin-Key)
- Add frontend API function `fetchGroupsWithCredentials` to call the new endpoint and parse S2A standard response format
- Add group fetching UI in S2AConfig edit mode with refresh button and loading state
- Implement batch group name caching in list view to display human-readable group names instead of IDs
- Deduplicate API requests by grouping profiles with identical credentials to avoid redundant calls
- Add fallback to display group ID when name is unavailable