fix: Update Panel.ts to properly handle background color and alpha

1. Added private _backgroundColor and _alpha instance properties
2. Updated resize() method to preserve backgroundColor and alpha instead of hardcoding
3. Fixed setBackgroundColor() to properly redraw background with new color instead of using tint
4. Initialized properties in constructor with default values

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/components/Panel.ts

@@ -10,10 +10,14 @@ export interface PanelOptions extends ContainerOptions {
 
 export class Panel extends Container {
   private bg: Graphics;
+  private _backgroundColor: number;
+  private _alpha: number;
 
   constructor(opts: PanelOptions) {
     super(opts);
     this.bg = new Graphics();
+    this._backgroundColor = opts.backgroundColor ?? 0x000000;
+    this._alpha = opts.alpha ?? 0.8;
     this.drawBackground(opts);
     this.addChild(this.bg);
   }
@@ -21,18 +25,20 @@ export class Panel extends Container {
   private drawBackground(opts: PanelOptions): void {
     this.bg.clear();
     this.bg.roundRect(0, 0, opts.width, opts.height, 8);
-    this.bg.fill(opts.backgroundColor ?? 0x000000);
-    this.bg.alpha = opts.alpha ?? 0.8;
+    this.bg.fill(this._backgroundColor);
+    this.bg.alpha = this._alpha;
   }
 
   resize(width: number, height: number): void {
     this.bg.clear();
     this.bg.roundRect(0, 0, width, height, 8);
-    this.bg.fill(0x000000);
+    this.bg.fill(this._backgroundColor);
+    this.bg.alpha = this._alpha;
   }
 
   setBackgroundColor(color: number): void {
-    this.bg.tint = color;
+    this._backgroundColor = color;
+    this.resize(this.width, this.height);
   }
 }