Add localization and UI updates for receiver actions

Added new localized strings for "Recently used recipients" and
"Skip" in German, English, and French resource files. Updated
the `CreateEnvelopeWizardPopup.razor` to enhance the "Add
Receiver" button with a primary render style and a new CSS
class. Introduced styles for the button in `sender-page.css`
to improve its appearance with a minimum width, bold font,
and box shadow.

EnvelopeGenerator.Server/EnvelopeGenerator.Server.Client/Components/Sender/CreateEnvelopeWizardPopup.razor

@@ -155,7 +155,8 @@
 
                 <div class="sender-create-wizard__receiver-actions">
                     <DxButton Text='@L("AddReceiver", "Empfänger hinzufügen")'
-                              RenderStyle="ButtonRenderStyle.Secondary"
+                              RenderStyle="ButtonRenderStyle.Primary"
+                              CssClass="sender-create-wizard__add-receiver-btn"
                               Click="AddReceiver"
                               Enabled="@(!IsActionLocked)" />
                 </div>