Add UI features, alarm ack, historian UTC fix, and Client.UI documentation

Major changes across the client stack:
- Settings persistence (connection, subscriptions, alarm source)
- Deferred OPC UA SDK init for instant startup
- Array/status code formatting, write value popup, alarm acknowledgment
- Severity-colored alarm rows, condition dedup on server side
- DateTimeRangePicker control with preset buttons and UTC text input
- Historian queries use wwTimezone=UTC and OPCQuality column
- Recursive subscribe from tree, multi-select remove
- Connection panel with expander, folder chooser for cert path
- Dynamic tab headers showing subscription/alarm counts
- Client.UI.md documentation with headless-rendered screenshots

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/ZB.MOM.WW.LmxOpcUa.Client.UI.Tests/Screenshots/ScreenshotTestApp.cs

@@ -0,0 +1,20 @@
+using Avalonia;
+using Avalonia.Headless;
+using Avalonia.Themes.Fluent;
+
+namespace ZB.MOM.WW.LmxOpcUa.Client.UI.Tests.Screenshots;
+
+public class ScreenshotTestApp : Application
+{
+    public override void Initialize()
+    {
+        Styles.Add(new FluentTheme());
+    }
+
+    public static AppBuilder BuildAvaloniaApp() => AppBuilder.Configure<ScreenshotTestApp>()
+        .UseSkia()
+        .UseHeadless(new AvaloniaHeadlessPlatformOptions
+        {
+            UseHeadlessDrawing = false
+        });
+}