frontend: add tests & stories for many components

Also more compound adoption
Also prettier upgrade

frontend/src/components/LoadingScreen/LoadingScreen.module.css

@@ -0,0 +1,21 @@
+/* Copyright 2023 The Matrix.org Foundation C.I.C.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+.loading-screen {
+    display: flex;
+    min-height: 100vh;
+    justify-content: center;
+    align-items: center;
+}