Update config key for PrefersColorScheme header name

Updated the configuration key from `PrefersColorScheme:HeaderName`
to `PrefersColorSchemeHeader:HeaderName` in the following files:
- PrefersColorSchemeMiddleware.cs
- _Layout.cshtml
- _LayoutAdmin.cshtml

This change ensures the application reads the header name from
the updated configuration section.

src/Moonglade.Web/Middleware/PrefersColorSchemeMiddleware.cs

@@ -4,7 +4,7 @@ public class PrefersColorSchemeMiddleware(RequestDelegate next)
 {
     public async Task InvokeAsync(HttpContext context, IConfiguration configuration)
     {
-        var headerName = configuration["PrefersColorScheme:HeaderName"];
+        var headerName = configuration["PrefersColorSchemeHeader:HeaderName"];
         if (string.IsNullOrWhiteSpace(headerName))
         {
             await next(context);

src/Moonglade.Web/Pages/Shared/_Layout.cshtml

@@ -30,7 +30,7 @@
     // This is to enhance user experience by stopping the screen from blinking when switching pages
     bool useServerSideDarkMode = false;
     bool usePrefersColorSchemeHeader = Configuration.GetSection("PrefersColorSchemeHeader:Enabled").Get<bool>();
-    var prefersColorScheme = Context.Request.Headers[Configuration["PrefersColorScheme:HeaderName"]!];
+    var prefersColorScheme = Context.Request.Headers[Configuration["PrefersColorSchemeHeader:HeaderName"]!];
     if (usePrefersColorSchemeHeader && prefersColorScheme == "dark")
     {
         useServerSideDarkMode = true;

src/Moonglade.Web/Pages/Shared/_LayoutAdmin.cshtml

@@ -11,7 +11,7 @@
     // This is to enhance user experience by stopping the screen from blinking when switching pages
     bool useServerSideDarkMode = false;
     bool usePrefersColorSchemeHeader = Configuration.GetSection("PrefersColorSchemeHeader:Enabled").Get<bool>();
-    var prefersColorScheme = Context.Request.Headers[Configuration["PrefersColorScheme:HeaderName"]!];
+    var prefersColorScheme = Context.Request.Headers[Configuration["PrefersColorSchemeHeader:HeaderName"]!];
     if (usePrefersColorSchemeHeader && prefersColorScheme == "dark")
     {
         useServerSideDarkMode = true;