Add support for invoice attachments

Introduced the `InvoiceAttachment` entity and its relationship with `ZugferdInvoice` to manage extracted invoice attachments. Updated `AppDbContext` and added a migration to create the `InvoiceAttachments` table with cascading delete behavior and an index for optimized queries.

Enhanced the UI to display attachments in `Details.cshtml`, including file type icons, file size, and extraction date. Added a new `ViewAttachment` page to render or download attachments based on their type, with support for XML, plain text, images, and downloads.

Implemented `AttachmentViewerService` to determine viewer types and MIME types for attachments. Registered the service in the DI container. Updated `Upload.cshtml.cs` to save extracted attachments to the database.

Improved user experience with syntax highlighting for XML files and appropriate messages for unsupported file types.

DXApp.TemplateKitProject/Migrations/20260602123528_AddInvoiceAttachments.cs

@@ -0,0 +1,51 @@
+using System;
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace DXApp.TemplateKitProject.Migrations
+{
+    /// <inheritdoc />
+    public partial class AddInvoiceAttachments : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.CreateTable(
+                name: "InvoiceAttachments",
+                columns: table => new
+                {
+                    Id = table.Column<int>(type: "int", nullable: false)
+                        .Annotation("SqlServer:Identity", "1, 1"),
+                    ZugferdInvoiceId = table.Column<int>(type: "int", nullable: false),
+                    OriginalFileName = table.Column<string>(type: "nvarchar(max)", nullable: true),
+                    SavedFilePath = table.Column<string>(type: "nvarchar(max)", nullable: true),
+                    FileSizeBytes = table.Column<long>(type: "bigint", nullable: false),
+                    IsZugferdXml = table.Column<bool>(type: "bit", nullable: false),
+                    ExtractedAt = table.Column<DateTime>(type: "datetime2", nullable: false)
+                },
+                constraints: table =>
+                {
+                    table.PrimaryKey("PK_InvoiceAttachments", x => x.Id);
+                    table.ForeignKey(
+                        name: "FK_InvoiceAttachments_ZugferdInvoices_ZugferdInvoiceId",
+                        column: x => x.ZugferdInvoiceId,
+                        principalTable: "ZugferdInvoices",
+                        principalColumn: "Id",
+                        onDelete: ReferentialAction.Cascade);
+                });
+
+            migrationBuilder.CreateIndex(
+                name: "IX_InvoiceAttachments_ZugferdInvoiceId",
+                table: "InvoiceAttachments",
+                column: "ZugferdInvoiceId");
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropTable(
+                name: "InvoiceAttachments");
+        }
+    }
+}