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/07-tests-and-validation.prompt.md

@@ -0,0 +1,80 @@
+---
+description: "Create xUnit tests for transfer consistency, balance calculations, carryover logic, and PDF export totals"
+agent: "agent"
+---
+
+# Step 7: Tests & Validation
+
+Create tests in `tests/Duempelkas.Tests/`.
+
+## Test 1: Transfer Consistency (`TransferServiceTests.cs`)
+
+### Scenario: CreateTransfer_CreatesLinkedExpenseAndIncome
+Given:
+- Account A and Account B exist
+- Both have AccountYear for 2026 with OpeningBalance = 0
+
+When:
+- A transfer of 100.00 EUR is created from A/2026 to B/2026 with title "Test Transfer"
+
+Then:
+- Exactly 2 entries are created in the database
+- Entry on A is `EntryType.Expense`, amount = 100.00
+- Entry on B is `EntryType.Income`, amount = 100.00
+- Both entries share the same date and title
+- Exactly 1 `TransferLink` exists linking the two entries
+- Account A total balance = -100.00
+- Account B total balance = +100.00
+
+### Scenario: DeleteTransfer_RemovesBothSides
+Given:
+- A transfer exists between A and B
+
+When:
+- The source entry is deleted via `IEntryService.DeleteEntryAsync`
+
+Then:
+- Both entries are removed
+- The TransferLink is removed
+- Account A and B balances return to 0
+
+## Test 2: Yearly Statement Calculation (`YearlyStatementCalculationTests.cs`)
+
+### Scenario: YearlySummary_CalculatesCorrectTotals
+Given:
+- Account with Year 2026, OpeningBalance = 500.00
+- Entries: Income 800.00, Income 400.00, Expense 200.00, Expense 250.00
+
+Then:
+- TotalIncome = 1200.00
+- TotalExpense = 450.00
+- YearlyMovement = 750.00
+- ClosingBalance = 1250.00
+
+### Scenario: YearlySummary_IncludesTransfersCorrectly
+Given:
+- Account A, Year 2026, OpeningBalance = 500.00
+- Income entry: 1000.00
+- Transfer out to Account B: 300.00 (creates Expense entry on A)
+
+Then:
+- A's TotalIncome = 1000.00
+- A's TotalExpense = 300.00
+- A's YearlyMovement = 700.00
+- A's ClosingBalance = 1200.00
+
+## Test Setup
+- Use EF Core `InMemory` or SQLite in-memory (`Data Source=:memory:`) for test database.
+- Wire up real service implementations against the test database.
+- Use `FluentAssertions` for readable assertions.
+- Arrange/Act/Assert pattern throughout.
+
+## Manual Verification Checklist (as code comments)
+1. Launch on Windows — app window opens with dashboard
+2. Launch on Linux — app window opens (requires WebKit2GTK)
+3. Create account → appears on dashboard with 0 balance
+4. Add year 2026 with carryover 500 → year appears in selector
+5. Add income/expense entries → table populates, balance updates
+6. Create transfer between accounts → both sides appear with transfer badge
+7. Export PDF → file saves, totals match on-screen values
+8. Delete transfer → both sides removed, balances correct