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>

EnvelopeGenerator.Application/Resources/Resource.en-US.resx

@@ -778,6 +778,8 @@
   <data name="SignedDocument" xml:space="preserve"><value>Signed document</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>Add document: click or drag and drop</value></data>
+  <data name="SenderWizard_DocumentReplaceHint" xml:space="preserve"><value>Click to replace or drag a file onto this area.</value></data>
   <data name="UnexpectedErrorTryAgainLater" xml:space="preserve"><value>An unexpected error occurred. Please try again later.</value></data>
   <data name="UploadPdf" xml:space="preserve"><value>Upload PDF</value></data>
   <data name="UserName" xml:space="preserve"><value>Username</value></data>

EnvelopeGenerator.Application/Resources/Resource.fr-FR.resx

@@ -778,6 +778,8 @@
   <data name="SignedDocument" xml:space="preserve"><value>Document signé</value></data>
   <data name="System" xml:space="preserve"><value>Système</value></data>
   <data name="SenderWizard_Step_Start" xml:space="preserve"><value>Démarrer</value></data>
+  <data name="SenderWizard_DocumentUploadSimpleHint" xml:space="preserve"><value>Ajouter un document : cliquer ou glisser-deposer</value></data>
+  <data name="SenderWizard_DocumentReplaceHint" xml:space="preserve"><value>Cliquez pour remplacer ou glissez un fichier sur cette zone.</value></data>
   <data name="UnexpectedErrorTryAgainLater" xml:space="preserve"><value>Une erreur inattendue s'est produite. Veuillez réessayer plus tard.</value></data>
   <data name="UploadPdf" xml:space="preserve"><value>Téléverser un PDF</value></data>
   <data name="UserName" xml:space="preserve"><value>Nom d'utilisateur</value></data>