Fix for v29: Add nation count loading for JoinPrivateLobbyModal; change HvN difficulty (#2941)

## Description:

1. In JoinPrivateLobbyModal the nation count loading was missing. That
caused the team preview UI to show different player counts compared to
the HostLobbyModal. For example it showed 0/0 nations for the
HumansVsNations team mode (instead of 2/2):

<img width="726" height="217" alt="Screenshot 2026-01-16 211337"
src="https://github.com/user-attachments/assets/8b4219de-e2b2-46ff-a600-c86915e5bdb3"
/>

2. Turn down HvN difficulty from Impossible to Hard. 
We steamrolled over Hard nations in the playtest (at least in two of the
three games) because we donated lots of troops to each other.
But after some API data research I noticed that only 33% of players in
public team games ever use the donate functionality.
And we probably have less skilled players in public games than in the
playtest.
So its probably better to use the Hard difficulty to ensure balanced
gameplay.
I know, I'm overthinking this 😂

## 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/server/MapPlaylist.ts

@@ -127,9 +127,7 @@ export class MapPlaylist {
       publicGameModifiers: { isCompact, isRandomSpawn, startingGold },
       startingGold,
       difficulty:
-        playerTeams === HumansVsNations
-          ? Difficulty.Impossible
-          : Difficulty.Easy,
+        playerTeams === HumansVsNations ? Difficulty.Hard : Difficulty.Easy,
       infiniteGold: false,
       infiniteTroops: false,
       maxTimerValue: undefined,