fix: Add missing frontend public files required for build

- Fix .gitignore excluding frontend/public directory
- Add index.html and manifest.json to version control
- Remove unnecessary favicon references
- These files are essential for React build process

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

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

frontend/public/manifest.json

@@ -0,0 +1,9 @@
+{
+  "short_name": "MinIO WebUI",
+  "name": "MinIO WebUI Management Portal",
+  "icons": [],
+  "start_url": ".",
+  "display": "standalone",
+  "theme_color": "#000000",
+  "background_color": "#ffffff"
+}