fix: 添加提取站点logo功能以防止favicon闪烁

[wucm667]

问题

页面刷新时，浏览器��加载 HTML 中的默认 /logo.png，
等待 Vue 启动后才通过 JS 替换为自定义 logo，导致闪烁。

解决方案

• 后端 embed_on.go: injectSettings() 现在接收 siteLogo 参数，
  在注入配置脚本前，先替换 \u003clink rel="icon"\u003e 的 href
• 前端 vite.config.ts: 开发模式插件同样替换 favicon，
  保持开发和生产行为一致

这样浏览器解析 HTML 时直接拿到正确的 favicon URL，
无需等待 JS 执行，完全消除闪烁。

[Copilot]

Pull request overview

This pull request adds functionality to inject the custom site logo into the HTML favicon tag during server-side rendering, preventing the default logo from flashing when the page loads. The change addresses a user experience issue where the browser would initially load the default /logo.png and then replace it with the custom logo after Vue.js initializes.

Changes:

• Backend: Adds extractSiteLogo() function to extract logo from settings via reflection, updates injectSettings() to replace the favicon tag in HTML before injection
• Frontend: Updates Vite development plugin to perform the same favicon replacement for development/production parity

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 10 comments.

File	Description
backend/internal/web/embed_on.go	Adds reflection-based logo extraction and favicon HTML replacement in server-side rendering
frontend/vite.config.ts	Adds favicon replacement in development mode to match production behavior

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