Add GitHub Copilot prompt files

- Step-by-step prompts for solution skeleton, domain model,
  DbContext mappings, application services, UI shell/pages,
  PDF export, and tests/validation

.github/prompts/04-application-services.prompt.md

@@ -0,0 +1,57 @@
+---
+description: "Define and implement application service interfaces and implementations for accounts, years, entries, transfers, balance queries, and file operations"
+agent: "agent"
+---
+
+# Step 4: Application Services
+
+## Interfaces (in `src/Duempelkas.App/Services/`)
+
+### IAccountService
+- `Task<List<AccountSummaryDto>> GetAllAccountsAsync()` — returns each account with its current total balance across all years.
+- `Task<AccountSummaryDto> CreateAccountAsync(string name)`
+- `Task RenameAccountAsync(int accountId, string newName)`
+- `Task DeleteAccountAsync(int accountId)`
+
+### IAccountYearService
+- `Task<List<AccountYearDto>> GetYearsForAccountAsync(int accountId)`
+- `Task<AccountYearDto> CreateYearAsync(int accountId, int year, decimal openingBalance)`
+- `Task<decimal> SuggestCarryoverAsync(int accountId, int year)` — calculates prior year's closing balance as suggestion.
+- `Task UpdateOpeningBalanceAsync(int accountYearId, decimal openingBalance)`
+
+### IEntryService
+- `Task<List<EntryDto>> GetEntriesAsync(int accountYearId)`
+- `Task<EntryDto> CreateEntryAsync(int accountYearId, EntryType type, DateTime date, string title, decimal amount)`
+- `Task CreateTransferAsync(int sourceAccountYearId, int targetAccountYearId, DateTime date, string title, decimal amount)` — atomically creates two entries + TransferLink.
+- `Task DeleteEntryAsync(int entryId)` — if linked transfer, deletes both sides atomically.
+- `Task UpdateEntryAsync(int entryId, DateTime date, string title, decimal amount)`
+
+### IBalanceQueryService
+- `Task<decimal> GetAccountTotalBalanceAsync(int accountId)` — OpeningBalance + sum(Income) - sum(Expense) across all years.
+- `Task<YearlySummaryDto> GetYearlySummaryAsync(int accountYearId)` — returns opening balance, total income, total expense, yearly movement, closing balance.
+
+### IPdfStatementService
+- `Task<byte[]> GenerateYearlyStatementAsync(int accountYearId)`
+
+### IFileSaveService
+- `Task<string?> SaveFileAsync(byte[] content, string suggestedFileName)` — abstracts native file-save dialog.
+
+## DTOs (in `src/Duempelkas.App/Services/Models/`)
+- `AccountSummaryDto` { Id, Name, TotalBalance, CreatedUtc }
+- `AccountYearDto` { Id, AccountId, Year, OpeningBalance }
+- `EntryDto` { Id, AccountYearId, Type, Date, Title, Amount, IsTransfer, TransferLinkId, LinkedAccountName }
+- `YearlySummaryDto` { OpeningBalance, TotalIncome, TotalExpense, YearlyMovement, ClosingBalance }
+
+## Implementations (in `src/Duempelkas.Infrastructure/Services/`)
+- Implement each interface using `FinanceDbContext`.
+- All balance calculations use the single formula: `OpeningBalance + sum(Income) - sum(Expense)`.
+- Transfer creation must be wrapped in a transaction.
+- Entry deletion must check for TransferLink and delete both sides if linked.
+
+## DI Registration
+- Create `src/Duempelkas.Infrastructure/DependencyInjection.cs` with `AddInfrastructure(this IServiceCollection, string connectionString)` extension method that registers DbContext and all services.
+
+## Conventions
+- File-scoped namespaces.
+- Async/await throughout.
+- No business logic in entities; all in services.