frontend: Migrate to jotai and urql

This cuts the bundle size by 50% and makes it easier to reason about state.
It removes the usage of react-router-dom and replaces it with a simple router atom based on jotai-location.
Since the screens will be quite simple, I don't expect that we'll need the advanced caching features of react-relay, hence the switch to urql.

frontend/src/graphql.ts

@@ -0,0 +1,25 @@
+// 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.
+
+import {
+  createClient,
+  dedupExchange,
+  cacheExchange,
+  fetchExchange,
+} from "@urql/core";
+
+export const client = createClient({
+  url: "/graphql",
+  exchanges: [dedupExchange, cacheExchange, fetchExchange],
+});