Add infrastructure layer with persistence and services

- EF Core DbContext and entity configurations
- Design-time factory for migrations
- Initial and soft-delete migrations
- Service implementations: Account, AccountYear, Entry,
  BalanceQuery, FileSave, PdfStatement
- Dependency injection registration

src/Duempelkas.Infrastructure/Services/FileSaveService.cs

@@ -0,0 +1,60 @@
+using System.Diagnostics;
+using System.Runtime.InteropServices;
+using Duempelkas.App.Services;
+
+namespace Duempelkas.Infrastructure.Services;
+
+public class FileSaveService : IFileSaveService
+{
+    public async Task<string?> SaveFileAsync(byte[] content, string suggestedFileName)
+    {
+        var filePath = await ShowSaveDialogAsync(suggestedFileName);
+        if (filePath == null) return null;
+
+        await File.WriteAllBytesAsync(filePath, content);
+        return filePath;
+    }
+
+    private static Task<string?> ShowSaveDialogAsync(string suggestedFileName)
+    {
+        return Task.Run(() =>
+        {
+            if (!RuntimeInformation.IsOSPlatform(OSPlatform.Windows))
+            {
+                // Fallback: save to Documents folder
+                var documentsPath = Environment.GetFolderPath(Environment.SpecialFolder.MyDocuments);
+                return Path.Combine(documentsPath, suggestedFileName);
+            }
+
+            var extension = Path.GetExtension(suggestedFileName);
+            var filterName = extension == ".pdf" ? "PDF-Dateien" : "Dateien";
+            var filter = $"{filterName} (*{extension})|*{extension}|Alle Dateien (*.*)|*.*";
+
+            var script = $@"
+Add-Type -AssemblyName System.Windows.Forms
+$dialog = New-Object System.Windows.Forms.SaveFileDialog
+$dialog.Filter = '{filter}'
+$dialog.FileName = '{suggestedFileName.Replace("'", "''")}'
+$dialog.Title = 'Datei speichern'
+if ($dialog.ShowDialog() -eq 'OK') {{ $dialog.FileName }} else {{ '' }}
+";
+
+            var psi = new ProcessStartInfo
+            {
+                FileName = "powershell",
+                Arguments = $"-NoProfile -Command \"{script.Replace("\"", "\\\"\"")}\"",
+                RedirectStandardOutput = true,
+                UseShellExecute = false,
+                CreateNoWindow = true
+            };
+
+            using var process = Process.Start(psi);
+            if (process == null) return null;
+
+            var result = process.StandardOutput.ReadToEnd().Trim();
+            process.WaitForExit();
+
+            return string.IsNullOrWhiteSpace(result) ? null : result;
+        });
+    }
+}