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/layout/ProjectNav.tsx

@@ -0,0 +1,45 @@
+const NAV_ITEMS = [
+  { href: '', label: '概要' },
+  { href: '/members', label: 'メンバー' },
+  { href: '/settings', label: '設定' },
+] as const;
+
+/**
+ * プロジェクト内サブナビゲーション。
+ * href は `/projects/{projectId}` を起点とする相対パス。
+ */
+export function ProjectNav({
+  projectId,
+  active,
+}: {
+  projectId: number;
+  active: 'overview' | 'members' | 'settings';
+}) {
+  const activeMap: Record<string, boolean> = {
+    overview: active === 'overview',
+    members: active === 'members',
+    settings: active === 'settings',
+  };
+
+  return (
+    <nav className="flex gap-4 border-b bg-white px-6 py-2 text-sm">
+      {NAV_ITEMS.map((item) => {
+        const key = item.href === '' ? 'overview' : item.href.slice(1);
+        const href = `/projects/${projectId}${item.href}`;
+        return (
+          <a
+            key={item.href}
+            href={href}
+            className={
+              activeMap[key]
+                ? 'font-medium text-blue-600'
+                : 'text-gray-600 hover:underline'
+            }
+          >
+            {item.label}
+          </a>
+        );
+      })}
+    </nav>
+  );
+}