[BUGFIX] allow users to update username pre-game (#3298)

## Description:

Fix player rename in pre-game lobby on rejoin

Previously, when a player left a lobby, changed their name, and
rejoined, the server reused the original Client object without updating
the username. Now rejoinClient accepts the new username and applies it
if the game hasn't started yet, while still preserving names mid-game
for consistency.


## 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:

w.o.n

src/server/Client.ts

@@ -17,7 +17,7 @@ export class Client {
     public readonly roles: string[] | undefined,
     public readonly flares: string[] | undefined,
     public readonly ip: string,
-    public readonly username: string,
+    public username: string,
     public readonly uncensoredUsername: string,
     public ws: WebSocket,
     public readonly cosmetics: PlayerCosmetics | undefined,