Address PR review feedback: fix duplicate env var crash, mislabeled button, unused import

- Dedup env var keys via GroupBy before ToDictionary to prevent ArgumentException
  when duplicate enabled keys exist in UserEnvironmentVariablesList
- Fix Add button in EnvVarsDialog incorrectly labeled as "Save" (Action_Save -> Action_New)
- Remove unused System.Text.Json import from CivitDetailsPageViewModel

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: mohnjiles

StabilityMatrix.Avalonia/ViewModels/CheckpointBrowser/CivitDetailsPageViewModel.cs

@@ -3,7 +3,6 @@
 using System.ComponentModel.DataAnnotations;
 using System.Globalization;
 using System.Reactive.Linq;
-using System.Text.Json;
 using AsyncAwaitBestPractices;
 using Avalonia.Controls;
 using Avalonia.Controls.Notifications;

StabilityMatrix.Avalonia/Views/Dialogs/EnvVarsDialog.axaml

@@ -61,7 +61,7 @@
                         Height="50"
                         Command="{Binding AddRowCommand}"
                         IconSource="Add"
-                        Label="{x:Static lang:Resources.Action_Save}" />
+                        Label="{x:Static lang:Resources.Action_New}" />
                     <ui:CommandBarButton
                         Width="45"
                         Height="50"

StabilityMatrix.Core/Models/Settings/Settings.cs

@@ -170,7 +170,8 @@ public IReadOnlyDictionary<string, string> EnvironmentVariables
             var userVars = UserEnvironmentVariablesList is { Count: > 0 }
                 ? UserEnvironmentVariablesList
                     .Where(kvp => kvp.IsEnabled && !string.IsNullOrWhiteSpace(kvp.Key))
-                    .ToDictionary(kvp => kvp.Key, kvp => kvp.Value, StringComparer.Ordinal)
+                    .GroupBy(kvp => kvp.Key, StringComparer.Ordinal)
+                    .ToDictionary(g => g.Key, g => g.Last().Value, StringComparer.Ordinal)
                 : UserEnvironmentVariables;
 
             if (userVars is null || userVars.Count == 0)