Renames Key property to PrimaryKey

Updates code to use `PrimaryKey` instead of `Key`
property in `SiteMapping` for identifying pages.

This change improves clarity and consistency by
using a more descriptive name for the primary key field.

Author: BenjaminMichaelis

EssentialCSharp.Web/Controllers/HomeController.cs

@@ -30,7 +30,7 @@ public IActionResult Index()
 
             ViewBag.PageTitle = siteMapping.IndentLevel is 0 ? siteMapping.ChapterTitle + " " + siteMapping.RawHeading : siteMapping.RawHeading;
             ViewBag.NextPage = FlipPage(siteMapping!.ChapterNumber, siteMapping.PageNumber, true);
-            ViewBag.CurrentPageKey = siteMapping.Key;
+            ViewBag.CurrentPageKey = siteMapping.PrimaryKey;
             ViewBag.PreviousPage = FlipPage(siteMapping.ChapterNumber, siteMapping.PageNumber, false);
             ViewBag.HeadContents = headHtml;
             ViewBag.Contents = html;

EssentialCSharp.Web/Extensions/SiteMappingListExtensions.cs

@@ -1,5 +1,4 @@
-using EssentialCSharp.Common;
-using System.Globalization;
+using System.Globalization;
 
 namespace EssentialCSharp.Web.Extensions;
 
@@ -58,7 +57,7 @@ public static class SiteMappingListExtensions
                         currentMappingCount++;
                     }
                     overallMappingCount++;
-                    if (siteMapping.Key == key)
+                    if (siteMapping.PrimaryKey == key)
                     {
                         currentPageFound = true;
                     }