feat(m4): project & member management with tests and e2e

- ProjectRepository, ProjectMemberRepository, minimal NotificationRepository
- ProjectService: create/update/archive/delete project, add/remove member,
  permission checks (isMember/admin), member-added notification, getDashboard
  skeleton, getMyProjects, getMemberRole
- projectValidator, API services factory
- API routes: projects CRUD, members list/add/remove
- Screens: dashboard (project list + create form), project overview, members,
  settings; layout (Header/Sidebar/ProjectNav) and project components
- Unit tests (Project/ProjectMember repositories, ProjectService),
  integration project-member-permission, e2e project-management

components/project/DashboardWidget.tsx

@@ -0,0 +1,24 @@
+/**
+ * ダッシュボードの各項目を表示するウィジェットコンテナ。
+ * M4では骨組みのみ。全項目の集計は M13 で完成させる。
+ */
+export function DashboardWidget({
+  title,
+  children,
+  empty,
+}: {
+  title: string;
+  children?: React.ReactNode;
+  empty?: string;
+}) {
+  return (
+    <section className="rounded-lg border bg-white p-4 shadow-sm">
+      <h2 className="mb-3 text-sm font-semibold text-gray-700">{title}</h2>
+      {children ? (
+        <div className="space-y-2">{children}</div>
+      ) : (
+        <p className="text-sm text-gray-400">{empty ?? 'データがありません'}</p>
+      )}
+    </section>
+  );
+}