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/IAccountService.cs

@@ -0,0 +1,13 @@
+using Duempelkas.App.Services.Models;
+
+namespace Duempelkas.App.Services;
+
+public interface IAccountService
+{
+    Task<List<AccountSummaryDto>> GetAllAccountsAsync();
+    Task<AccountSummaryDto> GetAccountAsync(int accountId);
+    Task<AccountSummaryDto> CreateAccountAsync(string name);
+    Task RenameAccountAsync(int accountId, string newName);
+    Task UpdateCarryoverAsync(int accountId, decimal carryoverBalance);
+    Task DeleteAccountAsync(int accountId);
+}