feat(mobile-upgrade): 添加自定义 favicon

- 新增蓝色背景白色 M 字母的 SVG favicon
- 更新 layout.tsx 引用新 favicon

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

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

frontend/mobile-upgrade/public/favicon.svg

@@ -0,0 +1,4 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 32 32">
+  <rect width="32" height="32" rx="6" fill="#3B82F6"/>
+  <text x="16" y="22" font-family="Arial, sans-serif" font-size="18" font-weight="bold" fill="white" text-anchor="middle">M</text>
+</svg>

frontend/mobile-upgrade/src/app/layout.tsx

@@ -5,6 +5,9 @@ import './globals.css'
 export const metadata: Metadata = {
   title: 'Mobile Upgrade Admin',
   description: 'Mobile App Version Management Admin Panel',
+  icons: {
+    icon: '/favicon.svg',
+  },
 }
 
 export default function RootLayout({