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>

README.md

@@ -142,6 +142,7 @@ gr/                      Galaxy repository docs, SQL queries, schema
 | [Status Dashboard](docs/StatusDashboard.md) | HTTP server, health checks, metrics reporting |
 | [Service Hosting](docs/ServiceHosting.md) | TopShelf, startup/shutdown sequence, error handling |
 | [Client CLI](docs/Client.CLI.md) | Connect, browse, read, write, subscribe, historyread, alarms, redundancy commands |
+| [Client UI](docs/Client.UI.md) | Avalonia desktop client: browse, subscribe, alarms, history, write values |
 | [Security](docs/security.md) | Transport security profiles, certificate trust, production hardening |
 | [Redundancy](docs/Redundancy.md) | Non-transparent warm/hot redundancy, ServiceLevel, paired deployment |