feat(sender): add create-envelope wizard with bootstrap handoff

Introduce a 4-step modal wizard on SenderPage for creating envelopes.

Validate required fields, support optional skip, receiver management, PDF upload, and keyboard shortcuts.

Persist wizard payload in sessionStorage via token-based bootstrap service and navigate to editor with bootstrapToken.

Register bootstrap storage service in both client and server DI containers and add dedicated sender wizard styling.

EnvelopeGenerator.Server/EnvelopeGenerator.Server.Client/Pages/SenderPage.razor

@@ -9,14 +9,17 @@
 @using EnvelopeGenerator.Application.Common.Dto.History
 @using EnvelopeGenerator.Application.Resources
 @using EnvelopeGenerator.Domain.Resources
+@using EnvelopeGenerator.Server.Client.Components.Sender
 @using Microsoft.Extensions.Localization
 @inject EnvelopeGenerator.Server.Client.Services.EnvelopeService EnvelopeService
 @inject EnvelopeGenerator.Server.Client.Services.DocumentService DocumentService
 @inject EnvelopeGenerator.Server.Client.Services.PdfRenderService PdfRenderService
+@inject EnvelopeGenerator.Server.Client.Services.EnvelopeTypeService EnvelopeTypeService
 @inject EnvelopeGenerator.Server.Client.Services.AuthService AuthService
 @inject EnvelopeGenerator.Server.Client.Services.ThemeService ThemeService
 @inject EnvelopeGenerator.Server.Client.Services.UserPreferencesService UserPreferencesService
 @inject EnvelopeGenerator.Server.Client.Services.CustomImagesService CustomImagesService
+@inject EnvelopeGenerator.Server.Client.Services.CreateEnvelopeBootstrapStorageService CreateEnvelopeBootstrapStorageService
 @inject ILogger<SenderPage> Logger
 @inject NavigationManager Navigation
 @inject IJSRuntime JSRuntime
@@ -414,6 +417,11 @@
 
 </div>
 
+<CreateEnvelopeWizardPopup Visible="@_createWizardVisible"
+                          VisibleChanged="OnCreateWizardVisibleChangedAsync"
+                          EnvelopeTypes="@_envelopeTypes"
+                          StartEditorRequested="StartEditorFromWizardAsync" />
+
 @* ── PDF Vorschau Popup ────────────────────────────────────────────────── *@
 <DxPopup @bind-Visible="@_previewVisible"
          HeaderText="@($"{L("Preview", "Vorschau")} — {_selectedEnvelope?.Title} (#{_selectedEnvelope?.Id})")"
@@ -567,6 +575,9 @@
     private string _deleteReason = string.Empty;
     private string? _deleteErrorMessage;
 
+    private bool _createWizardVisible = false;
+    private IReadOnlyList<EnvelopeTypeDto> _envelopeTypes = [];
+
     // Stable keys used to store/restore each tab's layout in UserPreferences.GridLayouts
     private const string GridKeyActive = "sender.active-envelopes";
     private const string GridKeyCompleted = "sender.completed-envelopes";
@@ -596,6 +607,16 @@
         _appLogo = logos.App;
         _companyLogo = logos.Company;
 
+        try
+        {
+            _envelopeTypes = (await EnvelopeTypeService.GetAsync()).ToList();
+        }
+        catch (Exception ex)
+        {
+            Logger.LogWarning(ex, "Failed to load envelope types for create wizard.");
+            _envelopeTypes = [];
+        }
+
         await LoadEnvelopesAsync();
     }
 
@@ -693,7 +714,30 @@
 
     void CreateEnvelope()
     {
-        Navigation.NavigateTo("/sender/editor");
+        _createWizardVisible = true;
+    }
+
+    Task OnCreateWizardVisibleChangedAsync(bool visible)
+    {
+        _createWizardVisible = visible;
+        return Task.CompletedTask;
+    }
+
+    async Task StartEditorFromWizardAsync(CreateEnvelopeWizardResult result)
+    {
+        var payload = new CreateEnvelopeBootstrapPayload(
+            Title: result.Title,
+            Message: result.Message,
+            EnvelopeTypeId: result.EnvelopeTypeId,
+            Language: result.Language,
+            Receivers: result.Receivers
+                .Select(x => new CreateEnvelopeBootstrapReceiver(x.FullName, x.EmailAddress))
+                .ToList(),
+            DocumentBytes: result.DocumentBytes,
+            DocumentFileName: result.DocumentFileName);
+
+        var token = await CreateEnvelopeBootstrapStorageService.StoreAsync(payload);
+        Navigation.NavigateTo($"/sender/editor?bootstrapToken={token}");
     }
 
     void EditEnvelope()