Fix singleplayer check (#1260)

## Description:

Fix a broken check for singleplayer that relied on the `any` type to
mask undefined field access.

## Please complete the following:

- [x] I have added screenshots for all UI updates
- [x] I process any text displayed to the user through translateText()
and I've added it to the en.json file
- [x] I have added relevant tests to the test directory
- [x] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced
- [x] I understand that submitting code with bugs that could have been
caught through manual testing blocks releases and new features for all
contributors

src/client/Main.ts

@@ -327,8 +327,8 @@ class Client {
     }
   }
 
-  private async handleJoinLobby(event: CustomEvent) {
-    const lobby = event.detail as JoinLobbyEvent;
+  private async handleJoinLobby(event: CustomEvent<JoinLobbyEvent>) {
+    const lobby = event.detail;
     console.log(`joining lobby ${lobby.gameID}`);
     if (this.gameStop !== null) {
       console.log("joining lobby, stopping existing game");
@@ -403,7 +403,7 @@ class Client {
           (ad as HTMLElement).style.display = "none";
         });
 
-        if (event.detail.gameConfig?.gameType !== GameType.Singleplayer) {
+        if (lobby.gameStartInfo?.config.gameType !== GameType.Singleplayer) {
           history.pushState(null, "", `#join=${lobby.gameID}`);
         }
       },