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/02-domain-model.prompt.md

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+---
+description: "Implement the EF Core entity classes and enums for Account, AccountYear, Entry, TransferLink, and EntryType"
+agent: "agent"
+---
+
+# Step 2: Domain Model
+
+Implement the domain entities under `src/Duempelkas.Domain`.
+
+## Entities
+
+### Account (`Entities/Account.cs`)
+- `int Id` (PK)
+- `string Name` (required, max 200)
+- `DateTime CreatedUtc`
+- `ICollection<AccountYear> AccountYears` (navigation)
+
+### AccountYear (`Entities/AccountYear.cs`)
+- `int Id` (PK)
+- `int AccountId` (FK → Account)
+- `int Year` (e.g. 2026)
+- `decimal OpeningBalance` — explicit carryover from the previous year, user-entered
+- `DateTime CreatedUtc`
+- `Account Account` (navigation)
+- `ICollection<Entry> Entries` (navigation)
+
+Constraint: unique on `(AccountId, Year)`.
+
+### Entry (`Entities/Entry.cs`)
+- `int Id` (PK)
+- `int AccountYearId` (FK → AccountYear)
+- `EntryType Type` — Income or Expense
+- `DateTime Date`
+- `string Title` (required, max 500)
+- `decimal Amount` — always positive; sign determined by Type
+- `int? TransferLinkId` (FK → TransferLink, nullable)
+- `DateTime CreatedUtc`
+- `AccountYear AccountYear` (navigation)
+- `TransferLink? TransferLink` (navigation)
+
+### TransferLink (`Entities/TransferLink.cs`)
+- `int Id` (PK)
+- `int SourceEntryId` (FK → Entry, unique)
+- `int TargetEntryId` (FK → Entry, unique)
+- `string? Note`
+- `DateTime CreatedUtc`
+- `Entry SourceEntry` (navigation)
+- `Entry TargetEntry` (navigation)
+
+Invariants enforced at service level:
+- SourceEntry.Type must be Expense
+- TargetEntry.Type must be Income
+- Both share the same Amount and Date
+
+### EntryType (`Enums/EntryType.cs`)
+```csharp
+public enum EntryType { Income = 0, Expense = 1 }
+```
+
+## Conventions
+- Use `decimal` for all monetary amounts.
+- Store amounts as positive; behaviour (add/subtract) comes from `EntryType`.
+- File-scoped namespaces, nullable enabled.