Add a prompt to launch Element X on mobile

This shows a bottom sheet on mobile asking the user whether they want to open the call in Element X, as soon as the page is loaded.

src/room/AppSelectionModal.tsx

@@ -0,0 +1,77 @@
+/*
+Copyright 2023 New Vector Ltd
+
+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 { FC, MouseEvent, useCallback, useMemo, useState } from "react";
+import { useTranslation } from "react-i18next";
+import { Button, Text } from "@vector-im/compound-web";
+import { ReactComponent as PopOutIcon } from "@vector-im/compound-design-tokens/icons/pop-out.svg";
+
+import { Modal } from "../NewModal";
+import { useRoomSharedKey } from "../e2ee/sharedKeyManagement";
+import { getRoomUrl } from "../matrix-utils";
+import styles from "./AppSelectionModal.module.css";
+import { editFragmentQuery } from "../UrlParams";
+
+interface Props {
+  roomId: string | null;
+}
+
+export const AppSelectionModal: FC<Props> = ({ roomId }) => {
+  const { t } = useTranslation();
+
+  const [open, setOpen] = useState(true);
+  const onBrowserClick = useCallback(
+    (e: MouseEvent) => {
+      e.preventDefault();
+      e.stopPropagation();
+      setOpen(false);
+    },
+    [setOpen]
+  );
+
+  const roomSharedKey = useRoomSharedKey(roomId ?? "");
+  const appUrl = useMemo(() => {
+    // If the room ID is not known, fall back to the URL of the current page
+    const url = new URL(
+      roomId === null
+        ? window.location.href
+        : getRoomUrl(roomId, roomSharedKey ?? undefined)
+    );
+    // Edit the URL so that it opens in embedded mode
+    url.hash = editFragmentQuery(url.hash, (params) => {
+      params.set("isEmbedded", "");
+      return params;
+    });
+
+    const result = new URL("element://call");
+    result.searchParams.set("url", url.toString());
+    return result.toString();
+  }, [roomId, roomSharedKey]);
+
+  return (
+    <Modal className={styles.modal} title={t("Select app")} open={open}>
+      <Text size="md" weight="semibold">
+        {t("Ready to join?")}
+      </Text>
+      <Button kind="secondary" onClick={onBrowserClick}>
+        {t("Continue in browser")}
+      </Button>
+      <Button as="a" href={appUrl} Icon={PopOutIcon}>
+        {t("Open in the app")}
+      </Button>
+    </Modal>
+  );
+};