Drop unused System.Linq using

Pre-existing dead using surfaced after the doc edits — no LINQ methods
are used in this file.

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

Author: erikdarlingdata

src/PlanViewer.App/Services/AppSettingsService.cs

@@ -2,7 +2,6 @@
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.IO;
-using System.Linq;
 using System.Text.Json;
 using System.Text.Json.Serialization;