Add ZUGFeRD parsing and invoice storage support

Enhanced `ZugferdInvoice` model with default string values to prevent nulls. Updated `Upload.cshtml` to display parsed invoice data.

Refactored `Upload.cshtml.cs` to handle ZUGFeRD XML parsing and database storage. Introduced `ImportedInvoice` property and buffered file processing with `MemoryStream`.

Extended `ZugferdParserService` to support ZUGFeRD v1, v1.0 FeRD, and v2/Factur-X. Added version-specific parsing methods and namespaces. Improved date and decimal parsing for robustness.

Added database migration (`20260522084606_InitialCreate`) to define `ZugferdInvoices` table. Updated migration snapshot to reflect schema changes.

Fixed localization issue in `Upload.cshtml.cs` error message.

DXApp.TemplateKitProject/Migrations/20260522084606_InitialCreate.Designer.cs

@@ -0,0 +1,79 @@
+// <auto-generated />
+using System;
+using DXApp.TemplateKitProject.Data;
+using Microsoft.EntityFrameworkCore;
+using Microsoft.EntityFrameworkCore.Infrastructure;
+using Microsoft.EntityFrameworkCore.Metadata;
+using Microsoft.EntityFrameworkCore.Migrations;
+using Microsoft.EntityFrameworkCore.Storage.ValueConversion;
+
+#nullable disable
+
+namespace DXApp.TemplateKitProject.Migrations
+{
+    [DbContext(typeof(AppDbContext))]
+    [Migration("20260522084606_InitialCreate")]
+    partial class InitialCreate
+    {
+        /// <inheritdoc />
+        protected override void BuildTargetModel(ModelBuilder modelBuilder)
+        {
+#pragma warning disable 612, 618
+            modelBuilder
+                .HasAnnotation("ProductVersion", "8.0.8")
+                .HasAnnotation("Relational:MaxIdentifierLength", 128);
+
+            SqlServerModelBuilderExtensions.UseIdentityColumns(modelBuilder);
+
+            modelBuilder.Entity("DXApp.TemplateKitProject.Models.ZugferdInvoice", b =>
+                {
+                    b.Property<int>("Id")
+                        .ValueGeneratedOnAdd()
+                        .HasColumnType("int");
+
+                    SqlServerPropertyBuilderExtensions.UseIdentityColumn(b.Property<int>("Id"));
+
+                    b.Property<string>("BuyerName")
+                        .HasColumnType("nvarchar(max)");
+
+                    b.Property<string>("CurrencyCode")
+                        .HasColumnType("nvarchar(max)");
+
+                    b.Property<string>("Iban")
+                        .HasColumnType("nvarchar(max)");
+
+                    b.Property<DateTime>("ImportedAt")
+                        .HasColumnType("datetime2");
+
+                    b.Property<DateTime>("InvoiceDate")
+                        .HasColumnType("datetime2");
+
+                    b.Property<string>("InvoiceNumber")
+                        .HasColumnType("nvarchar(max)");
+
+                    b.Property<string>("RawXml")
+                        .HasColumnType("nvarchar(max)");
+
+                    b.Property<string>("SellerName")
+                        .HasColumnType("nvarchar(max)");
+
+                    b.Property<string>("SellerTaxId")
+                        .HasColumnType("nvarchar(max)");
+
+                    b.Property<string>("SourceType")
+                        .HasColumnType("nvarchar(max)");
+
+                    b.Property<decimal>("TaxAmount")
+                        .HasColumnType("decimal(18,2)");
+
+                    b.Property<decimal>("TotalAmount")
+                        .HasColumnType("decimal(18,2)");
+
+                    b.HasKey("Id");
+
+                    b.ToTable("ZugferdInvoices");
+                });
+#pragma warning restore 612, 618
+        }
+    }
+}