fix: resolve connectivity issues for Android and Web builds

- Android: Added INTERNET permission and enabled cleartext traffic in AndroidManifest.xml.
- Web: Implemented and registered CORSMiddleware in backend to allow cross-origin requests.
- Flutter: Updated SettingsProvider to use 10.0.2.2 as default for Android Emulator for easier local testing.

backend/internal/api/router.go

@@ -11,6 +11,9 @@ func SetupRouter() *gin.Engine {
 
 	r := gin.Default()
 
+	// Use CORS middleware to allow web access
+	r.Use(CORSMiddleware())
+
 	// 清除代理信任警告 "[WARNING] You trusted all proxies"
 	r.SetTrustedProxies(nil)