复用图标按钮收口背景色入口

编辑器左下面板 dock 的画布背景色入口改为复用 PlatformIconButton。

背景色入口继续用色块作为 icon 展示当前画布底色，保持原菜单交互不变。

补充编辑器测试断言共享图标按钮原语，并更新 TRACKING。

src/components/image-editor/ImageCanvasEditorView.tsx

@@ -2657,18 +2657,18 @@ export function ImageCanvasEditorView() {
             onPointerDown={(event) => event.stopPropagation()}
           >
             <div className="image-canvas-editor__background-control">
-              <button
-                type="button"
-                aria-label="画布背景色"
+              <PlatformIconButton
+                label="画布背景色"
                 title="画布背景色"
                 aria-expanded={isBackgroundMenuOpen}
                 onClick={() => setIsBackgroundMenuOpen((open) => !open)}
-              >
-                <span
-                  className="image-canvas-editor__background-swatch-current"
-                  style={{ backgroundColor: canvasBackgroundColor }}
-                />
-              </button>
+                icon={
+                  <span
+                    className="image-canvas-editor__background-swatch-current"
+                    style={{ backgroundColor: canvasBackgroundColor }}
+                  />
+                }
+              />
               {isBackgroundMenuOpen ? (
                 <PlatformFloatingMenu
                   className="image-canvas-editor__background-menu"