feat: SMS 스캔 화면 리팩토링 및 MVC 패턴 적용

- SMS 스캔 화면을 컨트롤러/서비스/위젯으로 분리
- 코드 가독성 및 유지보수성 향상
- 새로운 다국어 지원 키 추가
- Git 커밋 가이드라인 문서화

lib/widgets/sms_scan/scan_loading_widget.dart

@@ -0,0 +1,36 @@
+import 'package:flutter/material.dart';
+import '../../theme/app_colors.dart';
+import '../../widgets/themed_text.dart';
+import '../../l10n/app_localizations.dart';
+
+class ScanLoadingWidget extends StatelessWidget {
+  const ScanLoadingWidget({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return Padding(
+      padding: const EdgeInsets.symmetric(horizontal: 16.0),
+      child: Center(
+        child: Column(
+          mainAxisAlignment: MainAxisAlignment.center,
+          children: [
+            CircularProgressIndicator(
+              valueColor: AlwaysStoppedAnimation<Color>(AppColors.primaryColor),
+            ),
+            const SizedBox(height: 16),
+            ThemedText(
+              AppLocalizations.of(context).scanningMessages,
+              forceDark: true,
+            ),
+            const SizedBox(height: 8),
+            ThemedText(
+              AppLocalizations.of(context).findingSubscriptions,
+              opacity: 0.7,
+              forceDark: true,
+            ),
+          ],
+        ),
+      ),
+    );
+  }
+}