Handle confirmation on popstate event if player is active in a game (#2777)

Please merge it into v29, since in that version the back navigation out
of a game is currently **broken** after switching from hash-based to
path-based routing via #2740

## Description:

Protect against players accidentally leaving an active game by pressing
the browser back button. Uses the same confirmation dialog as the game
exit button.

Partially handles issue #1877 (protects against back button, not closing
tab or editing the URL directly).

<img width="861" height="373" alt="image"
src="https://github.com/user-attachments/assets/167cc137-6df3-44a7-a594-91ffd904857d"
/>

Partial credit to PR #2141

## 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

## Please put your Discord username so you can be contacted if a bug or
regression is found:

deshack_82603

src/client/ClientGameRunner.ts

@@ -69,7 +69,7 @@ export function joinLobby(
   lobbyConfig: LobbyConfig,
   onPrestart: () => void,
   onJoin: () => void,
-): () => void {
+): (force?: boolean) => boolean {
   console.log(
     `joining lobby: gameID: ${lobbyConfig.gameID}, clientID: ${lobbyConfig.clientID}`,
   );
@@ -79,6 +79,8 @@ export function joinLobby(
 
   const transport = new Transport(lobbyConfig, eventBus);
 
+  let currentGameRunner: ClientGameRunner | null = null;
+
   let hasJoined = false;
 
   const onconnect = () => {
@@ -122,9 +124,15 @@ export function joinLobby(
         terrainLoad,
         terrainMapFileLoader,
       )
-        .then((r) => r.start())
+        .then((r) => {
+          currentGameRunner = r;
+          r.start();
+        })
         .catch((e) => {
           console.error("error creating client game", e);
+
+          currentGameRunner = null;
+
           const startingModal = document.querySelector(
             "game-starting-modal",
           ) as HTMLElement;
@@ -165,9 +173,19 @@ export function joinLobby(
     }
   };
   transport.connect(onconnect, onmessage);
-  return () => {
+  return (force: boolean = false) => {
+    if (!force && currentGameRunner?.shouldPreventWindowClose()) {
+      console.log("Player is active, prevent leaving game");
+
+      return false;
+    }
+
     console.log("leaving game");
+
+    currentGameRunner = null;
     transport.leaveGame();
+
+    return true;
   };
 }
 
@@ -256,6 +274,21 @@ export class ClientGameRunner {
     this.lastMessageTime = Date.now();
   }
 
+  /**
+   * Determines whether window closing should be prevented.
+   *
+   * Used to show a confirmation dialog when the user attempts to close
+   * the window or navigate away during an active game session.
+   *
+   * @returns {boolean} `true` if the window close should be prevented
+   * (when the player is alive in the game), `false` otherwise
+   * (when the player is not alive or doesn't exist)
+   */
+  public shouldPreventWindowClose(): boolean {
+    // Show confirmation dialog if player is alive in the game
+    return !!this.myPlayer?.isAlive();
+  }
+
   private async saveGame(update: WinUpdate) {
     if (this.myPlayer === null) {
       return;