Improve admin user approvals UX and guard last admin

[RonaldRonnie]

What I implemented now

Success/error toasts across admin user flows
Manual user creation (/admin/users/create)

Uses useToast to show:

Success toast: “User created – Temporary credentials have been generated.”
Error toast: “Failed to create user” with server error description.
Validation toast if no role is selected: “Role required”.
Pending registrations (PendingRegistrationsTable)

On approve:

Shows a success toast: “User approved – The user account has been created successfully.”
On error, shows a destructive toast with the backend error.
If role is missing, shows a destructive toast “Role required” and keeps the inline error message.

On reject:

Shows a success toast: “Registration rejected.”
Or destructive toast if the server returns an error.
Pending team members (PendingTeamMembersTable)

On approve:

Shows a success toast: “Team member approved – The user account has been created successfully.”
On error, shows a destructive toast with the backend error.
If role is missing, shows a destructive toast “Role required” and keeps the inline error.
Business rule: prevent removing the last active admin
In ExistingUsersTable for all admin-affecting actions:

Update role:

Before calling updateUserRole, checks how many active admins exist.
If the target user is the only active admin and the new role is not admin, it:

• Blocks the action.

Shows an inline error message.
Shows a toast: “Cannot change role – You cannot remove the last active admin. Create another admin first.”

• Disable user:

If trying to disable the only active admin, it:

• Blocks the action.

Shows a toast “Cannot disable user – You cannot disable the last active admin…”.

• Delete user:

If trying to delete the only active admin, it:

• Blocks the action.
• Shows a toast “Cannot delete user – You cannot delete the last active admin…”.
• Other users (non-admins, or admins when there are ≥2 active admins) can still be updated/disabled/deleted as before.
• Toasts on existing user actions
• Update role: toast “Role updated” on success, “Failed to update role” on error.
• Disable user: toast “User disabled” / “Failed to disable user”.
• Enable user: toast “User enabled” / “Failed to enable user”.
• Delete user: toast “User deleted” / “Failed to delete user”.

Closes: #78