fix: 修复CoursePage图片路径并添加manifest.json

- 修复 CoursePage.tsx 中所有剩余的图片路径
- 将双引号图片路径转换为使用 process.env.PUBLIC_URL
- 添加 manifest.json 文件解决 PWA 警告
- 确保所有图片在 GitHub Pages 上正确加载

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

public/manifest.json

@@ -0,0 +1,15 @@
+{
+  "short_name": "多Agent协作系统",
+  "name": "多Agent协作系统 - AI教育平台",
+  "icons": [
+    {
+      "src": "favicon.ico",
+      "sizes": "64x64 32x32 24x24 16x16",
+      "type": "image/x-icon"
+    }
+  ],
+  "start_url": ".",
+  "display": "standalone",
+  "theme_color": "#000000",
+  "background_color": "#ffffff"
+}