feat: complete legacy parity and modernize operator UI

tests/MBN_STOCK_WEBVIEW.LegacyBridge.Tests/LegacyBridgeProtocolTests.cs

@@ -457,6 +457,46 @@ public sealed class LegacyBridgeProtocolTests
         Assert.IsType<LegacyActivateTradingHaltActionIntent>(haltAction);
     }
 
+    [Fact]
+    public void ExpertProjection_PreservesNullBuyAmountAndAvailableCompositionAction()
+    {
+        var expert = new LegacyExpertWorkflowSnapshot(
+            1,
+            new LegacyExpertSearchSnapshot(
+                string.Empty,
+                LegacyExpertRequestStatus.Ready,
+                [],
+                false,
+                string.Empty),
+            null,
+            new LegacyExpertPreviewSnapshot(
+                LegacyExpertRequestStatus.Ready,
+                [new LegacyExpertRecommendationView(0, "005930", "Samsung", null)],
+                false,
+                5,
+                1,
+                20,
+                string.Empty),
+            [new LegacyExpertActionView(
+                LegacyExpertWorkflowController.ActionId,
+                "Expert",
+                true)],
+            string.Empty,
+            LegacyOperatorStatusKind.Warning);
+        var snapshot = new LegacyOperatorSnapshot(
+            1, string.Empty, [], null, [], [], null, string.Empty,
+            LegacyOperatorStatusKind.Warning, Expert: expert);
+
+        using var document = JsonDocument.Parse(LegacyBridgeProtocol.SerializeState(snapshot));
+        var projected = document.RootElement.GetProperty("payload").GetProperty("expert");
+        var preview = Assert.Single(
+            projected.GetProperty("preview").GetProperty("items").EnumerateArray());
+
+        Assert.Equal(JsonValueKind.Null, preview.GetProperty("buyAmount").ValueKind);
+        Assert.True(Assert.Single(projected.GetProperty("actions").EnumerateArray())
+            .GetProperty("isAvailable").GetBoolean());
+    }
+
     [Fact]
     public void ParseExpertInitialConsonantQuery_PreservesCompatibilityJamo()
     {