fix: add Router Found template to resolve Blazor routing error

Router component requires Found and NotFound child templates.
Adds RouteView with MainLayout as default layout and NotFound error page.

Fixes: "Router component requires a value for the parameter Found" error

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

src/dotnet/QuantEngine.Web/Components/App.razor

@@ -24,7 +24,19 @@
         <CascadingAuthenticationState>
             <Router AppAssembly="@typeof(App).Assembly"
                     AdditionalAssemblies="new[] { typeof(QuantEngine.Web.Client.Pages.Dashboard).Assembly }"
-                    OnNavigateAsync="@OnNavigateAsync" />
+                    OnNavigateAsync="@OnNavigateAsync">
+                <Found Context="routeData">
+                    <RouteView RouteData="@routeData" DefaultLayout="@typeof(QuantEngine.Web.Client.Layout.MainLayout)" />
+                    <FocusOnNavigate RouteData="@routeData" Selector="h1" />
+                </Found>
+                <NotFound>
+                    <PageTitle>페이지를 찾을 수 없음</PageTitle>
+                    <div class="alert alert-danger">
+                        <h3>404 - 페이지를 찾을 수 없습니다</h3>
+                        <p>요청하신 페이지가 존재하지 않습니다.</p>
+                    </div>
+                </NotFound>
+            </Router>
         </CascadingAuthenticationState>
     </div>