Improve document upload UX and refactor wizard layout

Enhanced the `CreateEnvelopeWizardPopup` component:
- Added a new grid layout (`sender-create-wizard__body-layout`) to separate main content and side panel.
- Introduced a collapsible side panel for document upload and preview.
- Replaced old upload logic with `PdfFileDropZone` and added support for displaying document metadata (name, size) and replacement hints.
- Added asynchronous document preview rendering with `EnsureDocumentPreviewPreparedAsync`.

Localization updates:
- Added new localized strings for document upload hints in `de-DE`, `en-US`, and `fr-FR`.

CSS updates:
- Styled the new layout and side panel with transitions.
- Updated `PdfFileDropZone` styles for different states (e.g., document uploaded).

Refactored code for better maintainability and improved user feedback during document upload and preview preparation.

EnvelopeGenerator.Application/Resources/Resource.de-DE.resx

@@ -778,6 +778,8 @@
   <data name="SignedDocument" xml:space="preserve"><value>Signiertes Dokument</value></data>
   <data name="System" xml:space="preserve"><value>System</value></data>
   <data name="SenderWizard_Step_Start" xml:space="preserve"><value>Start</value></data>
+  <data name="SenderWizard_DocumentUploadSimpleHint" xml:space="preserve"><value>Dokument hinzufuegen: klicken oder per Drag-and-Drop ablegen</value></data>
+  <data name="SenderWizard_DocumentReplaceHint" xml:space="preserve"><value>Zum Ersetzen klicken oder eine Datei auf diese Flaeche ziehen.</value></data>
   <data name="UnexpectedErrorTryAgainLater" xml:space="preserve"><value>Ein unerwarteter Fehler ist aufgetreten. Bitte versuchen Sie es später erneut.</value></data>
   <data name="UploadPdf" xml:space="preserve"><value>PDF hochladen</value></data>
   <data name="UserName" xml:space="preserve"><value>Benutzername</value></data>