fix: replace time.Sleep with poll loops in tests, fix a11y target=_blank violations

backend/internal/square/square_dev_test.go

@@ -76,22 +76,13 @@ func TestDevClient_CreateCheckout_PendingThenCompleted(t *testing.T) {
 		t.Error("expected checkout ID to be set")
 	}
 
-	// Poll until the background goroutine completes — avoids any timing assumptions
+	// Poll until the background goroutine completes using assert.Eventually
 	var completed *PaymentResult
-	for i := 0; i < 20; i++ {
-		completed, err = client.GetCheckout(ctx, result.ID)
-		if err == nil && completed.Status == "COMPLETED" {
-			break
-		}
-		time.Sleep(200 * time.Millisecond)
-	}
-	if err != nil {
-		t.Fatalf("GetCheckout failed: %v", err)
-	}
-
-	if completed.Status != "COMPLETED" {
-		t.Errorf("expected status COMPLETED, got %s", completed.Status)
-	}
+	assert.Eventually(t, func() bool {
+		var getErr error
+		completed, getErr = client.GetCheckout(ctx, result.ID)
+		return getErr == nil && completed.Status == "COMPLETED"
+	}, 5*time.Second, 100*time.Millisecond, "expected checkout to complete")
 
 	if completed.Amount != 8000 {
 		t.Errorf("expected amount 8000 (7500 + 500 tip), got %d", completed.Amount)
@@ -128,16 +119,11 @@ func TestDevClient_CreateCheckout_NoTip(t *testing.T) {
 	}
 
 	var completed *PaymentResult
-	for i := 0; i < 20; i++ {
-		completed, err = client.GetCheckout(ctx, result.ID)
-		if err == nil && completed.Status == "COMPLETED" {
-			break
-		}
-		time.Sleep(200 * time.Millisecond)
-	}
-	if err != nil {
-		t.Fatalf("GetCheckout failed: %v", err)
-	}
+	assert.Eventually(t, func() bool {
+		var getErr error
+		completed, getErr = client.GetCheckout(ctx, result.ID)
+		return getErr == nil && completed.Status == "COMPLETED"
+	}, 5*time.Second, 100*time.Millisecond, "expected checkout to complete")
 
 	if completed.Amount != 5000 {
 		t.Errorf("expected amount 5000 (no tip), got %d", completed.Amount)