Close private lobby when host leaves 🚪 (#3503)

## Description:

When the host of a private lobby disconnects before the game starts, the
lobby is now closed:

- **Server:** Detects host disconnection via `creatorPersistentID`
match, kicks all remaining clients with a `host_left` reason, and marks
the game as ended so it's cleaned up by the game manager and can no
longer be joined.
- **Client:** Participants receive an `alert()` saying "The host has
left the lobby." and their JoinLobbyModal is closed automatically via
the `leave-lobby` event.
- **Phase logic:** `phase()` now returns `Finished` for ended private
lobbies that haven't started, preventing the game from lingering in
`Lobby` state.

## 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:

FloPinguin

resources/lang/en.json

@@ -863,7 +863,8 @@
   },
   "kick_reason": {
     "duplicate_session": "Kicked from game (you may have been playing on another tab)",
-    "lobby_creator": "Kicked by lobby creator"
+    "lobby_creator": "Kicked by lobby creator",
+    "host_left": "The host has left the lobby."
   },
   "send_troops_modal": {
     "title_with_name": "Send Troops to {name}",

src/client/ClientGameRunner.ts

@@ -172,6 +172,15 @@ export function joinLobby(
             composed: true,
           }),
         );
+      } else if (message.error === "kick_reason.host_left") {
+        alert(translateText("kick_reason.host_left"));
+        document.dispatchEvent(
+          new CustomEvent("leave-lobby", {
+            detail: { lobby: lobbyConfig.gameID, cause: "host-left" },
+            bubbles: true,
+            composed: true,
+          }),
+        );
       } else {
         showErrorModal(
           message.error,

src/server/GameServer.ts

@@ -35,6 +35,7 @@ export enum GamePhase {
 
 const KICK_REASON_DUPLICATE_SESSION = "kick_reason.duplicate_session";
 const KICK_REASON_LOBBY_CREATOR = "kick_reason.lobby_creator";
+const KICK_REASON_HOST_LEFT = "kick_reason.host_left";
 const KICK_REASON_TOO_MUCH_DATA = "kick_reason.too_much_data";
 const KICK_REASON_INVALID_MESSAGE = "kick_reason.invalid_message";
 
@@ -542,6 +543,20 @@ export class GameServer {
       this.activeClients = this.activeClients.filter(
         (c) => c.clientID !== client.clientID,
       );
+      // Close lobby when host leaves before game starts
+      if (
+        !this._hasStarted &&
+        !this.isPublic() &&
+        client.persistentID === this.creatorPersistentID
+      ) {
+        this.log.info("Host left, closing lobby", {
+          gameID: this.id,
+        });
+        for (const c of [...this.activeClients]) {
+          this.kickClient(c.clientID, KICK_REASON_HOST_LEFT);
+        }
+        this._hasEnded = true;
+      }
     });
     client.ws.on("error", (error: Error) => {
       if ((error as any).code === "WS_ERR_UNEXPECTED_RSV_1") {
@@ -847,6 +862,8 @@ export class GameServer {
         } else {
           return GamePhase.Active;
         }
+      } else if (this._hasEnded) {
+        return GamePhase.Finished;
       } else {
         return GamePhase.Lobby;
       }