Introduced `ESignParams` for configurable e-signature settings,
including `ApplicationName` and `DefaultReason`. Updated
`DocReceiverElementDto` and `PdfSignOptionsDto` to support
new properties. Registered `ESignParams` in DI container.
Refactored `SignPdfByElementCommand` to include new fields
and improved error handling. Added `SignPdfByEnvelopeReceiverCommand`
to enable signing documents for envelope receivers.
Replaced DevExpress with PdfSharp for PDF rendering in
`ReceiverSignedPage.razor`, improving layout and flexibility.
Enhanced `PdfSignatureController` with a new endpoint for
signing documents by envelope key.
Added `DataIntegrityException` for better error handling.
Updated `appsettings.json` with `ESignParams` configuration.
