feat(ui): Application-level settings — theme, log level, version (#28c)

[jaybarden1]

Parent

Sub-issue of #28.

Summary

Add application-level settings content to the settings overlay: theme selection, runtime log level, and app version display.

Scope (~3 production files)

• SettingsViewModel — extend skeleton from #28a: Theme (Light/Dark/System), LogLevel (Debug/Information/Warning/Error), AppVersion (read-only); persist theme + log level via IConfiguration or app-level store
• SettingsView.axaml — theme dropdown, log level dropdown, read-only version label
• App.axaml.cs — apply theme change at runtime (Application.Current.RequestedThemeVariant); wire Serilog LoggingLevelSwitch so log level change takes effect immediately without restart

Acceptance criteria

• Theme toggle applies immediately to the running app
• Log level change takes effect immediately (no restart required)
• App version label shows current assembly version
• Theme and log level selections persist across restarts
• dotnet build zero warnings; dotnet test all pass

Depends on

• #28a (settings shell)