docs: add banner image to README

Use GitHub raw URL for NuGet package compatibility.

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

Author: JusterZhu

README.md

@@ -1,5 +1,9 @@
 # GeneralUpdate.Maui
 
+<p align="center">
+  <img src="https://raw.githubusercontent.com/GeneralLibrary/GeneralUpdate.Maui/main/imgs/banner.png" alt="GeneralUpdate.Maui Banner" />
+</p>
+
 [![GitHub Stars](https://img.shields.io/github/stars/GeneralLibrary/GeneralUpdate.Maui?style=flat-square)](https://github.com/GeneralLibrary/GeneralUpdate.Maui/stargazers)
 [![GitHub Forks](https://img.shields.io/github/forks/GeneralLibrary/GeneralUpdate.Maui?style=flat-square)](https://github.com/GeneralLibrary/GeneralUpdate.Maui/network/members)
 [![License](https://img.shields.io/badge/license-Apache%202.0-blue.svg?style=flat-square)](./LICENSE)