Add user membership management to Admin Group Management view

[Copilot]

The admin Group Management page previously only supported searching and deleting groups. This update adds in-place user membership management so an administrator can select a group and add/remove users directly from the same view.

• Group-focused membership controls

  • Added group selection state in GroupManagement and a dedicated membership panel for the selected group.
  • Surface current members and non-members as actionable lists tied to the selected group.

• Add/remove membership actions

  • Wired UI actions to existing application service methods:
    • AssignUserToGroup(...)
    • RemoveUserFromGroup(...)
  • Added guardrails for empty selection and idempotent operations (already assigned / already removed).

• State refresh + operator feedback

  • After each mutation, refreshes group/user membership data and preserves selected context.
  • Added concise success/error messaging for membership changes in the admin workflow.

• UI copy and flow adjustments

  • Updated Group Management wording from delete-only behavior to include membership administration.
  • Kept delete-group behavior intact while extending the page with membership operations.

<button class="btn btn-outline-primary btn-sm"
        @onclick="() => AddUserToGroupAsync(user.UserId, _selectedGroup!.GroupId)">
    Add
</button>

<button class="btn btn-outline-danger btn-sm"
        @onclick="() => RemoveUserFromGroupAsync(user.UserId, _selectedGroup!.GroupId)">
    Remove
</button>

Original prompt

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This section details on the original issue you should resolve

<issue_title>Enhance group management view to be able to add/remove users</issue_title>
<issue_description>Enhance the group management view so when an administrator selects a group, the should be able to add or remove users to the group.
</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Enhance group management view to be able to add/remove users #12

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