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

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,