Fix duplicate membership_id error in receptionist registration wizard

Add uniqueness check for membership_id per branch in step 1 validation,
preventing the DB unique constraint violation when a membership_id is reused.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/Livewire/Receptionist/NewRegistrationWizard.php

@@ -251,6 +251,17 @@ public function nextStep(): void
             $this->validate($rules, $this->messages());
         }
 
+        // Membership ID uniqueness check on step 1
+        if ($this->currentStep === 1 && $this->membership_type === 'member' && $this->membership_id) {
+            $exists = Participant::where('branch_id', $this->branchId)
+                ->where('membership_id', $this->membership_id)
+                ->exists();
+            if ($exists) {
+                $this->addError('membership_id', 'رقم العضوية مستخدم بالفعل لمشترك آخر في هذا الفرع');
+                return;
+            }
+        }
+
         // Duplicate check on step 2 (guardian) — if duplicates found, stay on step 2
         if ($this->currentStep === 2 && !$this->duplicateCheckDone) {
             $this->checkDuplicates();