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>

src/Client/ZB.MOM.WW.OtOpcUa.Client.UI/ViewModels/SubscriptionsViewModel.cs

@@ -2,6 +2,7 @@ using System.Collections.ObjectModel;
 using CommunityToolkit.Mvvm.ComponentModel;
 using CommunityToolkit.Mvvm.Input;
 using Opc.Ua;
+using Serilog;
 using ZB.MOM.WW.OtOpcUa.Client.Shared;
 using ZB.MOM.WW.OtOpcUa.Client.Shared.Models;
 using ZB.MOM.WW.OtOpcUa.Client.UI.Services;
@@ -13,9 +14,17 @@ namespace ZB.MOM.WW.OtOpcUa.Client.UI.ViewModels;
 /// </summary>
 public partial class SubscriptionsViewModel : ObservableObject
 {
+    private static readonly ILogger Logger = Log.ForContext<SubscriptionsViewModel>();
+
     private readonly IUiDispatcher _dispatcher;
     private readonly IOpcUaClientService _service;
 
+    /// <summary>
+    ///     Last user-visible status message — set when a subscribe/unsubscribe operation fails so the
+    ///     shell can surface the diagnostic instead of silently dropping the error. Cleared on success.
+    /// </summary>
+    [ObservableProperty] private string? _statusMessage;
+
     [ObservableProperty]
     [NotifyCanExecuteChangedFor(nameof(AddSubscriptionCommand))]
     [NotifyCanExecuteChangedFor(nameof(RemoveSubscriptionCommand))]
@@ -85,11 +94,13 @@ public partial class SubscriptionsViewModel : ObservableObject
             {
                 ActiveSubscriptions.Add(new SubscriptionItemViewModel(nodeIdStr, interval));
                 SubscriptionCount = ActiveSubscriptions.Count;
+                StatusMessage = null;
             });
         }
-        catch
+        catch (Exception ex)
         {
-            // Subscription failed; no item added
+            Logger.Warning(ex, "AddSubscription failed for {NodeId}", nodeIdStr);
+            _dispatcher.Post(() => StatusMessage = $"Subscribe failed for {nodeIdStr}: {ex.Message}");
         }
     }
 
@@ -116,9 +127,11 @@ public partial class SubscriptionsViewModel : ObservableObject
 
                 _dispatcher.Post(() => ActiveSubscriptions.Remove(item));
             }
-            catch
+            catch (Exception ex)
             {
-                // Unsubscribe failed for this item; continue with others
+                Logger.Warning(ex, "Unsubscribe failed for {NodeId}", item.NodeId);
+                _dispatcher.Post(() => StatusMessage = $"Unsubscribe failed for {item.NodeId}: {ex.Message}");
+                // Continue with the other items in the batch.
             }
         }
 
@@ -146,11 +159,13 @@ public partial class SubscriptionsViewModel : ObservableObject
             {
                 ActiveSubscriptions.Add(new SubscriptionItemViewModel(nodeIdStr, intervalMs));
                 SubscriptionCount = ActiveSubscriptions.Count;
+                StatusMessage = null;
             });
         }
-        catch
+        catch (Exception ex)
         {
-            // Subscription failed
+            Logger.Warning(ex, "AddSubscriptionForNode failed for {NodeId}", nodeIdStr);
+            _dispatcher.Post(() => StatusMessage = $"Subscribe failed for {nodeIdStr}: {ex.Message}");
         }
     }
 
@@ -186,9 +201,10 @@ public partial class SubscriptionsViewModel : ObservableObject
             foreach (var child in children)
                 await AddSubscriptionRecursiveAsync(child.NodeId, child.NodeClass, intervalMs, maxDepth, currentDepth + 1);
         }
-        catch
+        catch (Exception ex)
         {
-            // Browse failed for this node; skip it
+            Logger.Warning(ex, "Recursive browse failed for {NodeId}; skipping subtree", nodeIdStr);
+            _dispatcher.Post(() => StatusMessage = $"Browse failed for {nodeIdStr}: {ex.Message}");
         }
     }