feat(PlayerExecution): downgrade defense posts on capture (#1957)

## Description:

Closes https://github.com/openfrontio/OpenFrontIO/issues/1619.

On capture, defense posts will be downgraded.

On the live version this means defense posts will be destroyed, as
defense posts can only be level 1.

Misc. changes:
- added `decreaserLevel` helper
- cleaned up if/else in tick unit loop for clarity to avoid yet another
nested layer

Continuation of the stale PR,
https://github.com/openfrontio/OpenFrontIO/pull/1622.

## 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:

Discord username: `seekerreturns`

src/core/execution/PlayerExecution.ts

@@ -30,9 +30,14 @@ export class PlayerExecution implements Execution {
     this.player.units().forEach((u) => {
       const tileOwner = this.mg!.owner(u.tile());
       if (u.info().territoryBound) {
-        if (tileOwner.isPlayer()) {
+        if (tileOwner?.isPlayer()) {
           if (tileOwner !== this.player) {
-            this.mg!.player(tileOwner.id()).captureUnit(u);
+            if (u.type() === UnitType.DefensePost) {
+              this.mg!.player(tileOwner.id()).captureUnit(u);
+              u.decreaseLevel(this.mg!.player(tileOwner.id()));
+            } else {
+              this.mg!.player(tileOwner.id()).captureUnit(u);
+            }
           }
         } else {
           u.delete();