feat: add a loader for SPA javascript loading

frontend/app/spa-loading-template.html

@@ -0,0 +1,43 @@
+<!-- 
+    This component is used to show a loading spinner when the SPA is loading.
+    Source: https://github.com/barelyhuman/snips/blob/dev/pages/css-loader.md
+-->
+<div class="loader"></div>
+<style>
+.loader {
+  display: block;
+  position: fixed;
+  z-index: 1031;
+  top: 50%;
+  left: 50%;
+  transform: translate(-50%, -50%);
+  width: 18px;
+  height: 18px;
+  box-sizing: border-box;
+  border: solid 2px transparent;
+  border-top-color: #000;
+  border-left-color: #000;
+  border-bottom-color: #efefef;
+  border-right-color: #efefef;
+  border-radius: 50%;
+  -webkit-animation: loader 400ms linear infinite;
+  animation: loader 400ms linear infinite;
+}
+
+\@-webkit-keyframes loader {
+  0% {
+    -webkit-transform: translate(-50%, -50%) rotate(0deg);
+  }
+  100% {
+    -webkit-transform: translate(-50%, -50%) rotate(360deg);
+  }
+}
+\@keyframes loader {
+  0% {
+    transform: translate(-50%, -50%) rotate(0deg);
+  }
+  100% {
+    transform: translate(-50%, -50%) rotate(360deg);
+  }
+}
+</style>