Modernize UI with DevExpress icons and improved layout

Updated `Index.cshtml`:
- Added icons and a more descriptive project title.
- Introduced a new introductory paragraph explaining the app's purpose.
- Replaced the old alert section with a detailed workflow explanation.
- Added cards for invoice upload and summary navigation.
- Included a technology stack section with badges for key tools.

Updated `Details.cshtml`:
- Replaced text-based icons with DevExpress icons for titles, buttons, and attachments.
- Improved attachment handling with `<i>` tags and consistent icon usage.
- Updated "No attachments" message to include an icon.

These changes enhance the UI's visual consistency, usability, and professionalism.

DXApp.TemplateKitProject/Pages/Invoices/Details.cshtml

@@ -11,14 +11,14 @@
     }
 }
 
-<h2>📄 Rechnungsdetails</h2>
-<a href="/Invoices" class="btn btn-secondary mb-3">← Zurück zur Liste</a>
+<h2><i class="dx-icon-doc"></i> Rechnungsdetails</h2>
+<a href="/Invoices" class="btn btn-secondary mb-3"><i class="dx-icon-back"></i> Zurück zur Liste</a>
 
 @if (!string.IsNullOrEmpty(Model.Invoice?.ResultFilePath))
 {
     <button class="btn btn-primary mb-3 ms-2"
             onclick="openPdfViewer(@Model.Invoice.Id)">
-        📄 Ergebnis anzeigen
+        <i class="dx-icon-pdffile"></i> Ergebnis anzeigen
     </button>
 }
 
@@ -60,26 +60,26 @@ else
     @* Anhänge-Sektion *@
     @if (Model.Invoice.Attachments.Any())
     {
-        <h4 class="mt-4">📎 Anhänge (@Model.Invoice.Attachments.Count)</h4>
+        <h4 class="mt-4"><i class="dx-icon-attach"></i> Anhänge (@Model.Invoice.Attachments.Count)</h4>
         <div class="list-group">
             @foreach (var attachment in Model.Invoice.Attachments)
             {
-                var icon = attachment.IsZugferdXml ? "📋" : "📄";
+                var icon = "dx-icon-file";
                 var extension = System.IO.Path.GetExtension(attachment.OriginalFileName).ToLowerInvariant();
                 icon = extension switch
                 {
-                    ".xml" => "📋",
-                    ".pdf" => "📄",
-                    ".jpg" or ".jpeg" or ".png" or ".gif" => "🖼️",
-                    ".txt" => "📝",
-                    _ => "📎"
+                    ".xml" => "dx-icon-exportxlsx",
+                    ".pdf" => "dx-icon-pdffile",
+                    ".jpg" or ".jpeg" or ".png" or ".gif" => "dx-icon-image",
+                    ".txt" => "dx-icon-txtfile",
+                    _ => "dx-icon-file"
                 };
 
                 <a href="javascript:void(0);" 
                    class="list-group-item list-group-item-action d-flex justify-content-between align-items-center"
                    onclick="openAttachmentViewer('@attachment.OriginalFileName', '@Uri.EscapeDataString(attachment.SavedFilePath)', '@extension')">
                     <div>
-                        <span class="me-2">@icon</span>
+                        <i class="@icon me-2"></i>
                         <strong>@attachment.OriginalFileName</strong>
                         @if (attachment.IsZugferdXml)
                         {
@@ -98,7 +98,7 @@ else
     }
     else
     {
-        <h4 class="mt-4">📎 Anhänge</h4>
+        <h4 class="mt-4"><i class="dx-icon-attach"></i> Anhänge</h4>
         <div class="alert alert-info">Keine Anhänge extrahiert.</div>
     }