chore: 优化深色模式主题并新增返回顶部按钮

[joesdu]

📝 描述 (Description)

更改深色模式配色为 Dracula 主题，提升视觉效果和可读性。
调整滚动条、按钮、焦点阴影及 HTTP 方法块的样式。
新增返回顶部按钮，支持平滑滚动并动态显示/隐藏。
优化主题切换按钮，改用 SVG 图标并移至顶部导航栏。
改进代码结构和注释，增强可维护性。

🔗 相关问题 (Related Issues)

• Closes #
• Fixes #

🏷️ 变更类型 (Type of Change)

• 🐛 错误修复 (Bug fix) (non-breaking change which fixes an issue)
• ✨ 新功能 (New feature) (non-breaking change which adds functionality)
• 💥 破坏性变更 (Breaking change) (fix or feature that would cause existing functionality to not work as expected)
• 📚 文档更新 (Documentation update)
• 🔧 重构 (Refactor) (no functional changes)
• 🧪 测试 (Tests) (adding or updating tests)
• 🛠️ 构建/CI 变更 (Build/CI changes)
• 🎨 样式变更 (Style changes) (formatting, etc.)
• 🚀 性能改进 (Performance improvements)
• 🔒 安全改进 (Security improvements)

✅ 检查清单 (Checklist)

• 我的代码遵循项目的编码标准 (My code follows the project's coding standards)
• 我已经对自己的代码进行了自我审查 (I have performed a self-review of my own code)
• 我已经在难以理解的区域注释了我的代码 (I have commented my code, particularly in hard-to-understand areas)
• 我已经对文档进行了相应的更改 (I have made corresponding changes to the documentation)
• 我的更改不会产生新的警告或错误 (My changes generate no new warnings or errors)
• 我已经添加了证明我的修复有效或我的功能工作的测试 (I have added tests that prove my fix is effective or that my feature works)
• 新增和现有的单元测试在本地通过我的更改 (New and existing unit tests pass locally with my changes)
• 任何依赖变更都已合并并发布到下游模块 (Any dependent changes have been merged and published in downstream modules)

🧪 测试 (Testing)

• 单元测试 (Unit tests)
• 集成测试 (Integration tests)
• 手动测试 (Manual testing)
• 其他 (请指定) (Other (please specify)):

📸 截图 (如果适用) (Screenshots (if applicable))

🔍 附加说明 (Additional Notes)

[Copilot]

Pull Request Overview

This PR enhances the Swagger UI customization by replacing emoji icons with SVG graphics and adding a back-to-top button feature. The theme has also been updated from GitHub Dark to Dracula color scheme.

• Replaced emoji icons (🌙, ☀️) with scalable SVG icons for better cross-platform compatibility
• Added a new back-to-top button that appears when scrolling down
• Changed theme color scheme from GitHub Dark to Dracula Theme

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 5 comments.

File	Description
theme-toggle.js	Replaced emoji icons with SVG graphics for theme toggle button; added new back-to-top button functionality with scroll detection
custom.css	Updated color scheme from GitHub Dark to Dracula Theme; added styling for the back-to-top button; repositioned theme toggle button