Fix train was destroyed message spam (#2774)

## Description:

Trains are made of a primary unit (`TrainExecution.train`) followed by 6
cars (`TrainExecution.cars`). Currently when any of the units is
destroyed, a message "Your Train was destroyed" is shown. In worst case
scenario, when all cars are blasted by a nuke, 7 messages are displayed
to the user, but the train continues.

Since the actual logic is unaffected as long as the primary unit stays
alive, displaying messages is confusing. This PR fixes it. The message
is only displayed when the primary unit is destroyed. The following cars
are only there for fancy visuals.



## Screencast

##### Current - multiple messages for single train


https://github.com/user-attachments/assets/3df04c71-d899-4f68-af83-36c9d0ffa730

First nuke was a test.
Second nuke destroyed the entire train, prompting 7 messages.
Third nuke destroyed one full train and then some. Prompting many too
many messages.

##### Fixed - one message, only if train was fully destroyed


https://github.com/user-attachments/assets/1f3840a7-6c62-487d-af3a-82de39dad9e8

First train was only partially destroyed, no message was shown, delivery
mission completed A+.
Following 2 trains had the front engine destroyed and therefore were
promptly decommissioned.

## Please complete the following:

- [x] I have added screenshots for all UI updates
- [ ] I process any text displayed to the user through translateText()
and I've added it to the en.json file
- [ ] 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:

moleole

src/core/execution/TrainExecution.ts

@@ -14,8 +14,8 @@ import { TrainStation } from "../game/TrainStation";
 export class TrainExecution implements Execution {
   private active = true;
   private mg: Game | null = null;
-  private train: Unit | null = null;
-  private cars: Unit[] = [];
+  private train: Unit | null = null; // primary unit
+  private cars: Unit[] = []; // stored back to front
   private hasCargo: boolean = false;
   private currentTile: number = 0;
   private spacing = 2;
@@ -69,6 +69,7 @@ export class TrainExecution implements Execution {
     if (this.train === null) {
       throw new Error("Not initialized");
     }
+
     if (!this.train.isActive() || !this.activeSourceOrDestination()) {
       this.deleteTrain();
       return;
@@ -113,7 +114,7 @@ export class TrainExecution implements Execution {
     this.cars.push(
       this.player.buildUnit(UnitType.Train, tile, {
         targetUnit: this.destination.unit,
-        trainType: TrainType.Engine,
+        trainType: TrainType.TailEngine,
       }),
     );
     for (let i = 0; i < this.numCars; i++) {