Add checkered colors for defended borders (#691)

## Description:

The borders near a defense post are currently colored differently, but
too discreetly for the user.

I suggest coloring them with a checkerboard pattern.


![image](https://github.com/user-attachments/assets/8f414976-bada-4793-812a-10f28da911f5)

![image](https://github.com/user-attachments/assets/81261148-88e3-4498-9f29-27a6a201b6c5)


## Please complete the following:

- [x] I have added screenshots for all UI updates
- [x] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced
- [x] I understand that submitting code with bugs that could have been
caught through manual testing blocks releases and new features for all
contributors

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

IngloriousTom

src/client/graphics/layers/TerritoryLayer.ts

@@ -267,15 +267,13 @@ export class TerritoryLayer implements Layer {
           )
           .filter((u) => u.unit.owner() == owner).length > 0
       ) {
-        const useDefendedBorderColor = playerIsFocused
-          ? this.theme.focusedDefendedBorderColor()
-          : this.theme.defendedBorderColor(owner);
-        this.paintCell(
-          this.game.x(tile),
-          this.game.y(tile),
-          useDefendedBorderColor,
-          255,
-        );
+        const borderColors = this.theme.defendedBorderColors(owner);
+        const x = this.game.x(tile);
+        const y = this.game.y(tile);
+        const lightTile =
+          (x % 2 == 0 && y % 2 == 0) || (y % 2 == 1 && x % 2 == 1);
+        const borderColor = lightTile ? borderColors.light : borderColors.dark;
+        this.paintCell(x, y, borderColor, 255);
       } else {
         const useBorderColor = playerIsFocused
           ? this.theme.focusedBorderColor()