결재 템플릿 단계 적용 구현

- ApprovalTemplate 엔티티·DTO·원격 리포지토리 추가
- ApprovalController에 템플릿 로딩/적용 상태와 assignSteps 호출 연동
- ApprovalPage 단계 탭에 템플릿 선택 UI 및 적용 확인 다이얼로그 구현
- 템플릿 적용 단위 테스트와 IMPLEMENTATION_TASKS 현황 갱신

lib/widgets/components/page_header.dart

@@ -0,0 +1,58 @@
+import 'package:flutter/material.dart';
+import 'package:shadcn_ui/shadcn_ui.dart';
+
+/// 페이지 상단 타이틀/설명/액션을 일관되게 출력하는 헤더.
+class PageHeader extends StatelessWidget {
+  const PageHeader({
+    super.key,
+    required this.title,
+    this.subtitle,
+    this.leading,
+    this.actions,
+    this.trailing,
+  });
+
+  final String title;
+  final String? subtitle;
+  final Widget? leading;
+  final List<Widget>? actions;
+  final Widget? trailing;
+
+  @override
+  Widget build(BuildContext context) {
+    final theme = ShadTheme.of(context);
+
+    return Row(
+      crossAxisAlignment: CrossAxisAlignment.start,
+      children: [
+        if (leading != null) ...[
+          leading!,
+          const SizedBox(width: 16),
+        ],
+        Expanded(
+          child: Column(
+            crossAxisAlignment: CrossAxisAlignment.start,
+            children: [
+              Text(title, style: theme.textTheme.h2),
+              if (subtitle != null) ...[
+                const SizedBox(height: 6),
+                Text(subtitle!, style: theme.textTheme.muted),
+              ],
+            ],
+          ),
+        ),
+        if (actions != null && actions!.isNotEmpty) ...[
+          Wrap(
+            spacing: 12,
+            runSpacing: 12,
+            children: actions!,
+          ),
+        ],
+        if (trailing != null) ...[
+          const SizedBox(width: 16),
+          trailing!,
+        ],
+      ],
+    );
+  }
+}