feat: 完善课程模板通用能力

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/components/common/BackgroundContainer/index.ts

@@ -0,0 +1,7 @@
+export { BackgroundContainer } from "./BackgroundContainer";
+export type {
+	BackgroundContainerElement,
+	BackgroundContainerFit,
+	BackgroundContainerProps,
+	BackgroundContainerRepeat,
+} from "./BackgroundContainer.types";