Enable the @typescript-eslint/prefer-readonly eslint rule (#1859)

## Description:

Enable the `@typescript-eslint/prefer-readonly` eslint rule.

## 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

src/client/graphics/layers/StructureLayer.ts

@@ -31,10 +31,10 @@ type UnitRenderConfig = {
 export class StructureLayer implements Layer {
   private canvas: HTMLCanvasElement;
   private context: CanvasRenderingContext2D;
-  private unitIcons: Map<string, HTMLImageElement> = new Map();
-  private theme: Theme;
-  private tempCanvas: HTMLCanvasElement;
-  private tempContext: CanvasRenderingContext2D;
+  private readonly unitIcons: Map<string, HTMLImageElement> = new Map();
+  private readonly theme: Theme;
+  private readonly tempCanvas: HTMLCanvasElement;
+  private readonly tempContext: CanvasRenderingContext2D;
 
   // Configuration for supported unit types only
   private readonly unitConfigs: Partial<Record<UnitType, UnitRenderConfig>> = {
@@ -71,9 +71,9 @@ export class StructureLayer implements Layer {
   };
 
   constructor(
-    private game: GameView,
-    private eventBus: EventBus,
-    private transformHandler: TransformHandler,
+    private readonly game: GameView,
+    private readonly eventBus: EventBus,
+    private readonly transformHandler: TransformHandler,
   ) {
     this.theme = game.config().theme();
     this.tempCanvas = document.createElement("canvas");