style: define mb-6 and other margin utilities to fix vertical spacing in logbook entries list

client/src/App.css

@@ -472,10 +472,30 @@ body {
   gap: 16px;
 }
 
+.mt-2 {
+  margin-top: 8px;
+}
+
 .mt-4 {
   margin-top: 16px;
 }
 
+.mt-6 {
+  margin-top: 24px;
+}
+
+.mb-2 {
+  margin-bottom: 8px;
+}
+
+.mb-4 {
+  margin-bottom: 16px;
+}
+
+.mb-6 {
+  margin-bottom: 24px;
+}
+
 .list-section {
   display: flex;
   flex-direction: column;