Make page header fixed

ui/css/app.css

@@ -66,6 +66,19 @@ i {
   }
 }
 
+body {
+  height: 100%;
+  display: flex;
+  flex-direction: column;
+}
+
+#content-wrapper {
+  flex-grow: 1;
+  display: flex;
+  overflow-x: auto;
+  overflow-y: auto;
+}
+
 #header {
   background-color: #f2eddf;
   border-bottom: 1px solid var(--color1);
@@ -262,10 +275,6 @@ i {
   }
 }
 
-#content-wrapper {
-  display: flex;
-}
-
 #side-menu {
   width: 200px;
   min-width: 200px;