fix(client-ui): resolve Low code-review findings (Client.UI-003,004,006,009,010,011)

- Client.UI-003: wire Serilog properly per CLAUDE.md — console sink +
  rolling daily file sink in Program.Main, Log.CloseAndFlush in finally,
  per-VM Log.ForContext<> loggers.
- Client.UI-004: migrate the cert-store folder picker from the obsolete
  OpenFolderDialog to StorageProvider.OpenFolderPickerAsync (with
  TryGetFolderFromPathAsync seed + TryGetLocalPath extraction).
- Client.UI-006: surface formerly silent catch blocks via an observable
  StatusMessage on the Subscriptions / Alarms VMs that bubbles up into
  the shell's status bar; soft fallbacks log at Information level so
  hard failures stay distinguishable.
- Client.UI-009: docs/Client.UI.md now lists Standard Deviation in the
  Aggregate row of the Query Options table.
- Client.UI-010: removed the unused MinDateTimeProperty /
  MaxDateTimeProperty styled properties from DateTimeRangePicker.
- Client.UI-011: updated the cert-store TextBox watermark from the
  legacy AppData/LmxOpcUaClient/pki to the canonical
  AppData/OtOpcUaClient/pki.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/Client/ZB.MOM.WW.OtOpcUa.Client.UI.Tests/SubscriptionsViewModelTests.cs

@@ -276,6 +276,41 @@ public class SubscriptionsViewModelTests
         _service.SubscribeCallCount.ShouldBe(2);
     }
 
+    /// <summary>
+    /// Regression test for Client.UI-006 — when SubscribeAsync throws, the failure must be surfaced
+    /// to the operator via the view model's StatusMessage rather than silently swallowed.
+    /// </summary>
+    [Fact]
+    public async Task AddSubscription_OnFailure_SurfacesStatusMessage()
+    {
+        _vm.IsConnected = true;
+        _vm.NewNodeIdText = "ns=2;s=SomeNode";
+        _service.SubscribeException = new Exception("Permission denied");
+
+        await _vm.AddSubscriptionCommand.ExecuteAsync(null);
+
+        _vm.ActiveSubscriptions.ShouldBeEmpty();
+        _vm.StatusMessage.ShouldNotBeNullOrWhiteSpace();
+        _vm.StatusMessage.ShouldContain("Permission denied");
+    }
+
+    /// <summary>
+    /// Regression test for Client.UI-006 — silent swallow when adding a subscription for a node
+    /// (the context-menu helper) must also surface a status to the operator.
+    /// </summary>
+    [Fact]
+    public async Task AddSubscriptionForNodeAsync_OnFailure_SurfacesStatusMessage()
+    {
+        _vm.IsConnected = true;
+        _service.SubscribeException = new Exception("Bad node id");
+
+        await _vm.AddSubscriptionForNodeAsync("ns=2;s=ContextMenuNode");
+
+        _vm.ActiveSubscriptions.ShouldBeEmpty();
+        _vm.StatusMessage.ShouldNotBeNullOrWhiteSpace();
+        _vm.StatusMessage.ShouldContain("Bad node id");
+    }
+
     [Fact]
     public async Task AddSubscriptionRecursiveAsync_RecursesNestedObjects()
     {