feat: add ebg fading

[HardyNLee]

介绍

让 ebg 切换背景图片时有过渡

[gemini-code-assist]

Summary of Changes

Hello @HardyNLee, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

此拉取请求为 WebGAL 应用程序引入了背景图片切换时的平滑淡入淡出过渡效果。通过在 HTML 中添加一个叠加层元素，并修改 JavaScript 函数来动画化该叠加层的透明度，从而实现从旧背景到新背景的视觉过渡。此功能已集成到现有的背景设置逻辑中，以提供更流畅的用户体验。

Highlights

• 新增背景叠加层元素: 在 index.html 中添加了一个新的 div 元素 ebgOverlay，用于实现背景切换时的平滑过渡效果。
• 更新 setEbg 函数: setEbg 函数现在能够处理背景图片的淡入淡出动画。它会存储前一个背景图片，并使用 ebgOverlay 元素进行动画过渡。
• 集成动画时长: 背景切换的动画时长现在被传递给 setEbg 函数，确保过渡效果与游戏中的其他动画同步。

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[gemini-code-assist]

Code Review

这个 PR 旨在为 ebg（效果背景）切换背景图片时增加一个淡入淡出过渡效果。实现方式是在 HTML 中增加一个覆盖层元素，并通过 CSS 和 JavaScript 动画来控制新旧背景的交替。整体实现思路清晰，但有几处可以改进的地方，主要涉及 CSS 代码的重复和 TypeScript 模块中状态管理的方式，具体请看我的详细评论。

[gemini-code-assist]

这个新的 CSS 类 .html-body__effect-background-overlay 与现有的 .html-body__effect-background 类（第 14-25 行）之间存在大量重复的样式属性，例如 background-position, background-repeat, background-size, position 等。

代码重复会增加未来维护的难度，当需要修改这些共享样式时，不得不在多个地方进行同步修改，容易出错。

为了遵循 DRY (Don't Repeat Yourself) 原则，建议进行重构。一个可能的方案是创建一个通用的类来包含共享的样式，然后让 .html-body__effect-background 和 .html-body__effect-background-overlay 都使用这个通用类，从而消除重复。

[gemini-code-assist]

使用模块级别的变量 previousImageUrl 来存储状态，会使 setEbg 函数变为有状态的（stateful）函数，并且不是可重入的（non-reentrant）。

虽然在当前的使用场景下（可能只有一个地方调用它），这可能不会造成问题，但如果未来有多个独立的组件需要调用 setEbg，它们会共享 previousImageUrl 这个状态，可能导致非预期的行为和竞态条件。

为了提高代码的健壮性和可维护性，建议将状态管理与函数逻辑分离。例如，可以创建一个 EbgManager 类来封装状态和逻辑，或者修改 setEbg 函数，让调用者负责维护和传递上一个 URL，使其成为一个无状态的纯函数。