Add "Cancel" button and navigation to sender page

Added the `NavigationManager` service injection to the
`EnvelopeSenderEditorPage.razor` file to enable navigation.
Introduced a "Cancel" button in the toolbar, styled with
`pdf-toolbar__btn pdf-toolbar__btn--reset`, which triggers
the `Cancel` method on click. The `Cancel` method navigates
the user to the `/sender` route, improving the user experience
by providing a clear way to cancel the current operation.

EnvelopeGenerator.Server/EnvelopeGenerator.Server/Components/Pages/EnvelopeSenderEditorPage.razor

@@ -7,6 +7,7 @@
 @using EnvelopeGenerator.Server.Services
 @using Microsoft.AspNetCore.Components.Forms
 @inject IJSRuntime JSRuntime
+@inject NavigationManager NavigationManager
 @inject AppVersionService AppVersion
 @inject ILogger<EnvelopeSenderEditorPage> Logger
 @inject EnvelopeReceiverPageDataService ReceiverPageDataService
@@ -98,6 +99,12 @@
             @* Right: Buttons *@
             <div style="display: flex; align-items: center; gap: 0.5rem; flex-shrink: 0;">
 
+                <button class="pdf-toolbar__btn pdf-toolbar__btn--reset"
+                        @onclick="Cancel"
+                        title="Abbrechen">
+                    Abbrechen
+                </button>
+
                 @* PDF Upload *@
                 <label class="pdf-toolbar__btn"
                        title="PDF hochladen"
@@ -424,6 +431,8 @@
 
     void RemoveField(SignatureFieldDraft field) => _signatureFields.Remove(field);
 
+    void Cancel() => NavigationManager.NavigateTo("/sender");
+
     // ── Overlay click → add signature field ──
     async Task OnOverlayClickAsync(MouseEventArgs e)
     {