Add configurable home background image with blur and dim controls

[Copilot]

• Add browser/node/service worker globals to ESLint config to silence no-undef on platform APIs
• Remove unused variables flagged in settings and socket debugger
• Verify production build with npm run build
• Investigate and address remaining legacy lint warnings

Original prompt

给程序构建一个新的功能，允许用户在设置项（统一存在settings。js中）输入一个图片地址，在主页背景加一点毛玻璃的显示，允许用户自定义毛玻璃幅度和暗色程度，注意研究vue项目特点，美观。注意不要运行npmlint。

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[cloudflare-workers-and-pages]

Deploying classworks with    Cloudflare Pages

Latest commit:	aa2b28d
Status:	✅  Deploy successful!
Preview URL:	https://605dc08c.classworks.pages.dev
Branch Preview URL:	https://copilot-add-background-blur.classworks.pages.dev

View logs

[Sunwuyuan]

@copilot 无法构建，修复错误，想办法让我可以预览。

[Copilot]

@copilot 无法构建，修复错误，想办法让我可以预览。

已在 aa2b28d 内补充浏览器/Service Worker 全局并清理未使用变量，npm run build 已通过，可直接 npm install && npm run dev -- --host --port 4173 预览。截图：https://github.com/user-attachments/assets/3a885c1d-60c2-4814-9f28-2f6a6a8b96d0