헤더∙로그인 UI 정비하고 벤더 오류 메시지 안정화

- 로그인 화면 상단에 브랜드 아이콘과 안내 문구를 추가하고 카드/푸터 레이아웃을 정리

- 네비게이션 레일/앱바 구성 재배치, 테마 토글 상단 이동, 그라데이션/브랜드 아이콘 스타일 조정

- ScaffoldMessenger 의존성 문제를 피하도록 벤더 컨트롤러 에러 스낵바 호출 시점을 프레임 이후로 이연

- IMPLEMENTATION_TASKS.md에 이번 변경 내역을 요약하여 진행 상황을 문서화

lib/features/login/presentation/pages/login_page.dart

@@ -1,3 +1,5 @@
+import 'dart:math' as math;
+
 import 'package:flutter/material.dart';
 import 'package:go_router/go_router.dart';
 import 'package:shadcn_ui/shadcn_ui.dart';
@@ -64,81 +66,188 @@ class _LoginPageState extends State<LoginPage> {
     final theme = ShadTheme.of(context);
 
     return Scaffold(
-      body: Center(
-        child: ConstrainedBox(
-          constraints: const BoxConstraints(maxWidth: 460),
-          child: Padding(
-            padding: const EdgeInsets.all(24),
-            child: ShadCard(
-              title: Text('Superport v2 로그인', style: theme.textTheme.h3),
-              description: Text(
-                '사번 또는 이메일과 비밀번호를 입력하여 대시보드로 이동합니다.',
-                style: theme.textTheme.muted,
-              ),
-              child: Column(
-                crossAxisAlignment: CrossAxisAlignment.stretch,
-                children: [
-                  ShadInput(
-                    controller: idController,
-                    placeholder: const Text('사번 또는 이메일'),
-                    autofillHints: const [AutofillHints.username],
-                    leading: const Icon(LucideIcons.user),
-                  ),
-                  const SizedBox(height: 16),
-                  ShadInput(
-                    controller: passwordController,
-                    placeholder: const Text('비밀번호'),
-                    obscureText: true,
-                    autofillHints: const [AutofillHints.password],
-                    leading: const Icon(LucideIcons.lock),
-                  ),
-                  const SizedBox(height: 12),
-                  Row(
-                    children: [
-                      ShadSwitch(
-                        value: rememberMe,
-                        onChanged: (value) =>
-                            setState(() => rememberMe = value),
-                      ),
-                      const SizedBox(width: 12),
-                      Text('자동 로그인', style: theme.textTheme.small),
-                    ],
-                  ),
-                  const SizedBox(height: 24),
-                  if (errorMessage != null)
-                    Padding(
-                      padding: const EdgeInsets.only(bottom: 12),
-                      child: Text(
-                        errorMessage!,
-                        style: theme.textTheme.small.copyWith(
-                          color: theme.colorScheme.destructive,
-                        ),
-                      ),
-                    ),
-                  ShadButton(
-                    onPressed: isLoading ? null : _handleSubmit,
-                    child: Row(
-                      mainAxisAlignment: MainAxisAlignment.center,
-                      mainAxisSize: MainAxisSize.min,
-                      children: [
-                        if (isLoading) ...[
-                          const SizedBox(
-                            width: 16,
-                            height: 16,
-                            child: CircularProgressIndicator(strokeWidth: 2),
-                          ),
-                          const SizedBox(width: 8),
-                        ],
-                        const Text('로그인'),
-                      ],
-                    ),
-                  ),
-                ],
-              ),
+      backgroundColor: theme.colorScheme.background,
+      body: SafeArea(
+        child: Center(
+          child: SingleChildScrollView(
+            padding: const EdgeInsets.symmetric(horizontal: 24, vertical: 40),
+            child: Column(
+              mainAxisAlignment: MainAxisAlignment.center,
+              children: [
+                _buildHeader(theme),
+                const SizedBox(height: 32),
+                ConstrainedBox(
+                  constraints: const BoxConstraints(maxWidth: 460),
+                  child: _buildLoginCard(theme),
+                ),
+                const SizedBox(height: 24),
+                _buildFooter(theme),
+              ],
             ),
           ),
         ),
       ),
     );
   }
+
+  /// 로그인 헤더에 브랜드 아이콘과 안내 문구를 배치한다.
+  Widget _buildHeader(ShadThemeData theme) {
+    return Column(
+      children: [
+        DecoratedBox(
+          decoration: BoxDecoration(
+            shape: BoxShape.circle,
+            gradient: LinearGradient(
+              colors: [
+                theme.colorScheme.primary,
+                theme.colorScheme.primary.withValues(alpha: 0.8),
+                theme.colorScheme.primary.withValues(alpha: 0.6),
+              ],
+              begin: Alignment.topLeft,
+              end: Alignment.bottomRight,
+            ),
+            boxShadow: [
+              BoxShadow(
+                color: theme.colorScheme.primary.withValues(alpha: 0.3),
+                blurRadius: 24,
+                offset: const Offset(0, 12),
+              ),
+            ],
+          ),
+          child: Padding(
+            padding: const EdgeInsets.all(20),
+            child: TweenAnimationBuilder<double>(
+              duration: const Duration(milliseconds: 1400),
+              tween: Tween(begin: 0, end: 1),
+              curve: Curves.easeInOut,
+              builder: (context, value, child) {
+                return Transform.rotate(
+                  angle: value * 2 * math.pi * 0.08,
+                  child: child,
+                );
+              },
+              child: Icon(
+                LucideIcons.ship,
+                size: 48,
+                color: theme.colorScheme.primaryForeground,
+              ),
+            ),
+          ),
+        ),
+        const SizedBox(height: 24),
+        Text(
+          'Superport v2',
+          style: theme.textTheme.h2.copyWith(
+            foreground: Paint()
+              ..shader = LinearGradient(
+                colors: [
+                  theme.colorScheme.primary,
+                  theme.colorScheme.primary.withValues(alpha: 0.7),
+                ],
+              ).createShader(const Rect.fromLTWH(0, 0, 220, 80)),
+          ),
+        ),
+        const SizedBox(height: 8),
+        Text(
+          '포트 운영 전용 스마트 ERP',
+          style: theme.textTheme.muted,
+          textAlign: TextAlign.center,
+        ),
+      ],
+    );
+  }
+
+  /// 로그인 폼 카드 레이아웃을 구성한다.
+  Widget _buildLoginCard(ShadThemeData theme) {
+    return ShadCard(
+      title: Text('Superport v2 로그인', style: theme.textTheme.h3),
+      description: Text(
+        '사번 또는 이메일과 비밀번호를 입력하여 대시보드로 이동합니다.',
+        style: theme.textTheme.muted,
+      ),
+      child: Column(
+        crossAxisAlignment: CrossAxisAlignment.stretch,
+        children: [
+          ShadInput(
+            controller: idController,
+            placeholder: const Text('사번 또는 이메일'),
+            autofillHints: const [AutofillHints.username],
+            leading: const Icon(LucideIcons.user),
+          ),
+          const SizedBox(height: 16),
+          ShadInput(
+            controller: passwordController,
+            placeholder: const Text('비밀번호'),
+            obscureText: true,
+            autofillHints: const [AutofillHints.password],
+            leading: const Icon(LucideIcons.lock),
+          ),
+          const SizedBox(height: 12),
+          Row(
+            children: [
+              ShadSwitch(
+                value: rememberMe,
+                onChanged: (value) => setState(() => rememberMe = value),
+              ),
+              const SizedBox(width: 12),
+              Text('자동 로그인', style: theme.textTheme.small),
+            ],
+          ),
+          const SizedBox(height: 24),
+          if (errorMessage != null)
+            Padding(
+              padding: const EdgeInsets.only(bottom: 12),
+              child: Text(
+                errorMessage!,
+                style: theme.textTheme.small.copyWith(
+                  color: theme.colorScheme.destructive,
+                ),
+              ),
+            ),
+          ShadButton(
+            onPressed: isLoading ? null : _handleSubmit,
+            child: Row(
+              mainAxisAlignment: MainAxisAlignment.center,
+              mainAxisSize: MainAxisSize.min,
+              children: [
+                if (isLoading) ...[
+                  const SizedBox(
+                    width: 16,
+                    height: 16,
+                    child: CircularProgressIndicator(strokeWidth: 2),
+                  ),
+                  const SizedBox(width: 8),
+                ],
+                const Text('로그인'),
+              ],
+            ),
+          ),
+        ],
+      ),
+    );
+  }
+
+  /// 로그인 하단에 간단한 안내 메시지를 노출한다.
+  Widget _buildFooter(ShadThemeData theme) {
+    return Column(
+      children: [
+        Row(
+          mainAxisAlignment: MainAxisAlignment.center,
+          children: const [
+            Icon(LucideIcons.packagePlus, size: 18),
+            SizedBox(width: 12),
+            Icon(LucideIcons.packageMinus, size: 18),
+            SizedBox(width: 12),
+            Icon(LucideIcons.handshake, size: 18),
+          ],
+        ),
+        const SizedBox(height: 16),
+        Text(
+          '입고 · 출고 · 대여 전 과정을 한 곳에서 관리하세요.',
+          style: theme.textTheme.small,
+          textAlign: TextAlign.center,
+        ),
+      ],
+    );
+  }
 }

lib/features/masters/vendor/presentation/pages/vendor_page.dart

@@ -116,10 +116,17 @@ class _VendorEnabledPageState extends State<_VendorEnabledPage> {
     final error = _controller.errorMessage;
     if (error != null && error != _lastError && mounted) {
       _lastError = error;
-      ScaffoldMessenger.of(
-        context,
-      ).showSnackBar(SnackBar(content: Text(error)));
-      _controller.clearError();
+      // 스캐폴드 메시지는 프레임 빌드 이후 안전하게 호출한다.
+      WidgetsBinding.instance.addPostFrameCallback((_) {
+        if (!mounted) {
+          return;
+        }
+        final messenger = ScaffoldMessenger.maybeOf(context);
+        if (messenger != null) {
+          messenger.showSnackBar(SnackBar(content: Text(error)));
+        }
+        _controller.clearError();
+      });
     }
   }