Add customer logo area to action bar

Introduced a customer logo section in `EnvelopeSenderPage.razor` that dynamically displays a customer logo or a placeholder when no logo is configured. The logo is fetched from the `CustomImagesService` and styled using new CSS classes in `sender-page.css`.

- Added `_companyLogo` property to store customer logo configuration.
- Updated `CustomImagesService` to fetch both app and customer logos.
- Modified `appsettings.json` to include logo paths and styles.
- Enhanced CSS for a three-section action bar layout.
- Added placeholder styling for missing customer logos.
- Updated initialization logic to load logos dynamically.
- Documented implementation details in `SF-74-PLAN.md`.

The implementation is modular and deployment-specific, allowing logo paths and styles to be updated via configuration without code changes.

EnvelopeGenerator.Server/EnvelopeGenerator.Server.Client/Pages/EnvelopeSenderPage.razor

@@ -41,6 +41,22 @@
                 <div class="sender-title">Umschlag-Übersicht</div>
             </div>
 
+            @* ── Kunden-Logo (Mitte) ───────────────────────────────────────── *@
+            <div class="sender-customer-logo-area">
+                @if (_companyLogo is not null && !string.IsNullOrEmpty(_companyLogo.Src))
+                {
+                    <img src="@_companyLogo.Src"
+                         style="height: @_companyLogo.Height"
+                         class="@_companyLogo.GetClass("Show")"
+                         alt="Kunden-Logo" />
+                }
+                else
+                {
+                    <div class="sf-customer-logo-placeholder">Kunden-Logo</div>
+                }
+            </div>
+
+            @* ── Toolbar (rechts) ─────────────────────────────────────────── *@
             <div class="sender-toolbar">
                 <button class="sender-btn sender-btn--primary" @onclick="CreateEnvelope" title="Neuen Umschlag erstellen">
                     <svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" fill="currentColor" viewBox="0 0 16 16">
@@ -355,6 +371,7 @@
 
     // Logo config loaded from /api/CustomImages after auth
     private EnvelopeGenerator.Server.Client.Services.CustomImageDto? _appLogo;
+    private EnvelopeGenerator.Server.Client.Services.CustomImageDto? _companyLogo;
 
     // Stable keys used to store/restore each grid's layout in UserPreferences.GridLayouts
     private const string GridKeyActive    = "sender.active-envelopes";
@@ -377,7 +394,8 @@
 
         // Load logo config — authenticated, so this should succeed
         var logos = await CustomImagesService.GetAsync();
-        _appLogo = logos.App;
+        _appLogo     = logos.App;
+        _companyLogo = logos.Company;
 
         await LoadEnvelopesAsync();
     }