Refactored `IPersonalContractingPartyRepository` to replace `GetByRegisterId` with `GetByNationalId`. Updated `InstitutionContractApplication` to use the new method and removed duplicate `RegisterId` validation logic. Removed `Index.cshtml` and `Index.cshtml.cs` files, including all associated HTML, scripts, styles, and backend logic, indicating a deprecation or restructuring of the related functionality.