Simplify wizard flow and improve document upload UX

Reduced wizard steps from 5 to 4 by merging document upload into step 3. Added a new localized hint (`SenderWizard_FinalUploadHint`) to guide users during the final document upload. Removed redundant document preview functionality and associated UI elements.

Refactored the document upload area to always remain visible on the right side of the wizard, using the `PdfFileDropZone` component. Removed the "Skip" button for optional steps and cleaned up unused methods, properties, and CSS classes.

Updated step titles and descriptions to reflect the new flow. General code cleanup to simplify logic and improve maintainability.

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

@@ -787,6 +787,7 @@
   <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="SenderWizard_FinalUploadHint" xml:space="preserve"><value>Letzte Moeglichkeit im Wizard: Bitte laden Sie jetzt Ihr PDF ueber den rechten Upload-Bereich hoch. Im Editor koennen Sie spaeter ebenfalls ein Dokument hochladen oder ersetzen.</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

@@ -787,6 +787,7 @@
   <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="SenderWizard_FinalUploadHint" xml:space="preserve"><value>Last chance in the wizard: please upload your PDF now using the upload area on the right. In the editor, you can still upload or replace a document later.</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

@@ -787,6 +787,7 @@
   <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="SenderWizard_FinalUploadHint" xml:space="preserve"><value>Derniere possibilite dans l'assistant : veuillez televerser votre PDF maintenant via la zone d'upload a droite. Dans l'editeur, vous pourrez encore televerser ou remplacer un document plus tard.</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>