WBS-11 BFF Hardening: Implement auth caching state provider for instant sidebar transitions, forward proxy authentication cookies, and resolve WASM server-side prerendering BaseAddress null exception

src/dotnet/QuantEngine.Web/Client/Infrastructure/CustomAuthenticationStateProvider.cs

@@ -16,6 +16,8 @@ namespace QuantEngine.Web.Client.Infrastructure
         private const string RoleKey = "quant_admin_role";
         private const string RememberUsernameKey = "quant_admin_remember_username";
 
+        private AuthenticationState? _cachedState;
+
         public CustomAuthenticationStateProvider(LocalStorageService localStorage, HttpClient http, IJSRuntime jsRuntime)
         {
             _localStorage = localStorage;
@@ -25,6 +27,12 @@ namespace QuantEngine.Web.Client.Infrastructure
 
         public override async Task<AuthenticationState> GetAuthenticationStateAsync()
         {
+            if (_cachedState != null && _cachedState.User.Identity?.IsAuthenticated == true)
+            {
+                Console.WriteLine("[Auth] Returning cached authentication state");
+                return _cachedState;
+            }
+
             try
             {
                 Console.WriteLine("[Auth] GetAuthenticationStateAsync called");
@@ -62,7 +70,9 @@ namespace QuantEngine.Web.Client.Infrastructure
                                 new Claim(ClaimTypes.Role, role ?? "Admin")
                             }, "QuantAdminAuth");
 
-                            return new AuthenticationState(new ClaimsPrincipal(identity));
+                            var state = new AuthenticationState(new ClaimsPrincipal(identity));
+                            _cachedState = state;
+                            return state;
                         }
                         else
                         {
@@ -106,7 +116,9 @@ namespace QuantEngine.Web.Client.Infrastructure
                                 new Claim(ClaimTypes.Role, role ?? "Admin")
                             }, "QuantAdminAuth");
 
-                            return new AuthenticationState(new ClaimsPrincipal(identity));
+                            var state = new AuthenticationState(new ClaimsPrincipal(identity));
+                            _cachedState = state;
+                            return state;
                         }
                     }
                 }
@@ -122,7 +134,8 @@ namespace QuantEngine.Web.Client.Infrastructure
                 Console.WriteLine($"[Auth] Unexpected error: {ex.Message}");
             }
 
-            return new AuthenticationState(_anonymous);
+            _cachedState = new AuthenticationState(_anonymous);
+            return _cachedState;
         }
 
         public async Task MarkUserAsAuthenticatedAsync(string username, string accessToken, string role)
@@ -152,7 +165,9 @@ namespace QuantEngine.Web.Client.Infrastructure
             }, "QuantAdminAuth");
 
             var user = new ClaimsPrincipal(identity);
-            NotifyAuthenticationStateChanged(Task.FromResult(new AuthenticationState(user)));
+            var state = new AuthenticationState(user);
+            _cachedState = state;
+            NotifyAuthenticationStateChanged(Task.FromResult(state));
         }
 
         public async Task MarkUserAsLoggedOutAsync()
@@ -164,7 +179,8 @@ namespace QuantEngine.Web.Client.Infrastructure
             {
                 await _localStorage.DeleteAsync(UsernameKey);
             }
-            NotifyAuthenticationStateChanged(Task.FromResult(new AuthenticationState(_anonymous)));
+            _cachedState = new AuthenticationState(_anonymous);
+            NotifyAuthenticationStateChanged(Task.FromResult(_cachedState));
         }
 
         public async Task LogoutFromServerAsync()