Fix 2 HvN UI bugs 🔧 (#3378)

## Description:

I noticed two HvN bugs.

1. Private lobbies don't set `maxPlayers` in `GameConfig`, causing
`getGameModeLabel()` to render "0 Humans vs 0 Nations". Fall back to the
simple "Humans vs Nations" label when `maxPlayers` is unavailable.

<img width="239" height="84" alt="Screenshot 2026-03-07 034150"
src="https://github.com/user-attachments/assets/b2f01b96-674f-47dc-ae03-06bec71e3134"
/>

2. In public HumansVsNations games, the server matches the nation count
to the human player count at game start. The lobby team size preview
wasn't reflecting this - it displayed the raw config value instead.
Added `isPublicGame` prop to `LobbyPlayerView` and an
`effectiveNationCount` getter that overrides the displayed nation count
to match `clients.length` only for public HvN games. Private lobby hosts
retain full slider control. (This bug got introduced with my
"Configurable nation count" PR)

## 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/JoinLobbyModal.ts

@@ -130,6 +130,8 @@ export class JoinLobbyModal extends BaseModal {
                         .lobbyCreatorClientID=${hostClientID}
                         .currentClientID=${this.currentClientID}
                         .teamCount=${this.gameConfig?.playerTeams ?? 2}
+                        .isPublicGame=${this.gameConfig?.gameType ===
+                        GameType.Public}
                         .nationCount=${nationsConfigToSlider(
                           this.gameConfig?.nations ?? "default",
                           this.nationCount,