Rework trains to encourage alliances (#1697)

## Description:

This update encourages alliances by modifying how train-based trading
works. Trading is now restricted to allied networks, and allied trades
offer better rewards than self-trades, incentivizing cooperation.

Changes:
- Train destinations are now limited to stations owned by the player or
their allies.
    - Delivering a train to one’s own station grants 4,000 gold.
- Delivering a train to an ally’s station grants 5,000 gold to both the
sender and the receiver.

This system encourages players to form alliances, especially with
factory-focused players.


## 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
- [x] I have read and accepted the CLA agreement (only required once).

## Please put your Discord username so you can be contacted if a bug or
regression is found:

IngloriousTom

src/core/configuration/DefaultConfig.ts

@@ -329,7 +329,7 @@ export class DefaultConfig implements Config {
     return Math.min(1400, Math.round(20 * Math.pow(numberOfStations, 0.5)));
   }
   trainGold(): Gold {
-    return BigInt(10_000);
+    return BigInt(4_000);
   }
   trainStationMinRange(): number {
     return 15;