feat: adopt material 3 theme and billing adjustments

lib/controllers/detail_screen_controller.dart

@@ -12,6 +12,8 @@ import 'package:intl/intl.dart';
 import '../widgets/dialogs/delete_confirmation_dialog.dart';
 import '../widgets/common/snackbar/app_snackbar.dart';
 import '../l10n/app_localizations.dart';
+import '../utils/billing_date_util.dart';
+import '../utils/business_day_util.dart';
 
 /// DetailScreen의 비즈니스 로직을 관리하는 Controller
 class DetailScreenController extends ChangeNotifier {
@@ -407,7 +409,14 @@ class DetailScreenController extends ChangeNotifier {
     subscription.monthlyCost = monthlyCost;
     subscription.websiteUrl = websiteUrl;
     subscription.billingCycle = _billingCycle;
-    subscription.nextBillingDate = _nextBillingDate;
+    // 선택일이 오늘(또는 과거)이면 결제 주기에 맞춰 다음 회차로 보정하여 저장
+    final originalDateOnly = DateTime(
+        _nextBillingDate.year, _nextBillingDate.month, _nextBillingDate.day);
+    var adjustedNext =
+        BillingDateUtil.ensureFutureDate(originalDateOnly, _billingCycle);
+    // 주말/고정 공휴일 보정 → 다음 영업일로 이월
+    adjustedNext = BusinessDayUtil.nextBusinessDay(adjustedNext);
+    subscription.nextBillingDate = adjustedNext;
     subscription.categoryId = _selectedCategoryId;
     subscription.currency = _currency;
 
@@ -433,6 +442,14 @@ class DetailScreenController extends ChangeNotifier {
         '이벤트활성=${subscription.isEventActive}');
 
     // 구독 업데이트
+    // 자동 보정이 발생했으면 안내
+    if (adjustedNext.isAfter(originalDateOnly)) {
+      AppSnackBar.showInfo(
+        context: context,
+        message: '다음 결제 예정일로 저장됨',
+      );
+    }
+
     await provider.updateSubscription(subscription);
 
     if (context.mounted) {
@@ -575,15 +592,5 @@ class DetailScreenController extends ChangeNotifier {
     return colors[hash % colors.length];
   }
 
-  /// 그라데이션 가져오기
-  LinearGradient getGradient(Color baseColor) {
-    return LinearGradient(
-      colors: [
-        baseColor,
-        baseColor.withValues(alpha: 0.8),
-      ],
-      begin: Alignment.topLeft,
-      end: Alignment.bottomRight,
-    );
-  }
+  // getGradient 제거됨 (그라데이션 미사용)
 }