feat(credits): exchange balance for points

Replace the legacy points-to-balance and external points purchase flows
with direct integer balance-to-points exchange. Enforce exchange and
points-transfer limits and capability checks across the UI and REST API.

inc/helpers/user-center-renderers/profile-credits.php

@@ -42,6 +42,10 @@ if ($show_credits) {
 	require $dir . '/credits-signin.php';
 }
 
+if ($show_wallet || $show_credits) {
+	require $dir . '/transfer-modal.php';
+}
+
 // Tabbed ledgers (Zibll record tabs).
 if ($show_wallet || $show_credits) :
 	$active_record = $show_wallet ? 'balance' : 'points';