fix(gro-279): show Pay Now button during staff impersonation

Remove !readOnly guard from Pay Now button and PaymentModal in
BillingPayments.tsx. The readOnly guard was too broad, hiding the
Pay Now button during all impersonation sessions even though
impersonation is the only way staff can access a client's billing
page. Destructive actions (remove payment method, autopay toggle)
still respect the readOnly flag.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

apps/web/src/portal/sections/BillingPayments.tsx

@@ -110,14 +110,12 @@ export function BillingPayments({ sessionId, readOnly }: BillingPaymentsProps) {
               {pending.length} unpaid invoice{pending.length > 1 ? "s" : ""}
             </p>
           </div>
-          {!readOnly && (
-            <button
+          <button
               onClick={() => setShowPaymentModal(true)}
               className="px-6 py-2 bg-(--color-accent) text-white rounded-lg text-sm font-medium hover:bg-(--color-accent-hover)"
             >
               Pay Now
             </button>
-          )}
         </div>
       )}
 
@@ -293,7 +291,7 @@ export function BillingPayments({ sessionId, readOnly }: BillingPaymentsProps) {
       )}
 
       {/* Payment Modal */}
-      {showPaymentModal && !readOnly && (
+      {showPaymentModal && (
         <PaymentModal
           pending={pending}
           totalPending={totalPending}