Add Blazor application layer with UI components and pages

- Service interfaces and DTO models
- Dashboard page with account overview
- Account detail page with year/entry management
- Reusable components: AccountCard, EntryTable, YearSelector
- Dialog components: Add/Edit Account, Entry, Transfer, Year
- Main layout and routing configuration

src/Duempelkas.App/Services/IFileSaveService.cs

@@ -0,0 +1,6 @@
+namespace Duempelkas.App.Services;
+
+public interface IFileSaveService
+{
+    Task<string?> SaveFileAsync(byte[] content, string suggestedFileName);
+}