Optimize post-auth calls with parallel execution

Refactored the code to improve performance by parallelizing independent post-authentication service calls using `Task.WhenAll`. This reduces the initial page load time by avoiding redundant HTTP calls and reusing pre-fetched data.

- Moved `ThemeService.LoadFromServerAsync` to a separate task.
- Parallelized service calls for theme, logos, envelope types, and envelopes.
- Introduced `ApplyEnvelopeResultAsync` to process envelope data.
- Updated `LoadEnvelopesAsync` to reuse pre-fetched envelope data.
- Removed redundant HTTP calls to streamline initialization logic.

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

@@ -618,22 +618,29 @@
             return;
         }
 
-        // Load theme + DisplayName from server after authentication.
-        // Non-fatal: if this fails the page still loads with default theme.
-        // ThemeService.ScheduleSaveAsync() already logs errors internally.
+        // Fan out all independent post-auth calls in parallel to reduce initial load time.
+        var themeTask = Task.Run(async () =>
+        {
         try { await ThemeService.LoadFromServerAsync(); }
         catch { /* swallow — default theme remains active */ }
+        });
 
-        _canUseReceiverEnvelopeUrl = await EnvelopeReceiverService.GetReceiverEnvelopeUrlFeatureEnabledAsync();
+        var featureTask = EnvelopeReceiverService.GetReceiverEnvelopeUrlFeatureEnabledAsync();
+        var logosTask = CustomImagesService.GetAsync();
+        var typesTask = EnvelopeTypeService.GetAsync();
+        var envelopesTask = EnvelopeService.GetAsync(includeDocAndDocRes: false);
 
-        // Load logo config — authenticated, so this should succeed
-        var logos = await CustomImagesService.GetAsync();
+        await Task.WhenAll(themeTask, featureTask, logosTask, typesTask, envelopesTask);
+
+        _canUseReceiverEnvelopeUrl = await featureTask;
+
+        var logos = await logosTask;
         _appLogo = logos.App;
         _companyLogo = logos.Company;
 
         try
         {
-            _envelopeTypes = (await EnvelopeTypeService.GetAsync()).ToList();
+            _envelopeTypes = (await typesTask).ToList();
         }
         catch (Exception ex)
         {
@@ -641,7 +648,8 @@
             _envelopeTypes = [];
         }
 
-        await LoadEnvelopesAsync();
+        // Use already-fetched envelope data instead of a second HTTP call.
+        await ApplyEnvelopeResultAsync(envelopesTask);
     }
 
     async Task LoadEnvelopesAsync()
@@ -649,10 +657,15 @@
         _isLoading = true;
         _errorMessage = null;
         await InvokeAsync(StateHasChanged);
+        await ApplyEnvelopeResultAsync(EnvelopeService.GetAsync(includeDocAndDocRes: false));
+    }
 
+    async Task ApplyEnvelopeResultAsync(Task<IEnumerable<EnvelopeDto>?> envelopesTask)
+    {
+        _isLoading = true;
         try
         {
-            _allEnvelopes = await EnvelopeService.GetAsync(includeDocAndDocRes: false) ?? [];
+            _allEnvelopes = (await envelopesTask) ?? [];
 
             // Split into active and completed based on effective status
             // (history-aware: rejected envelopes should not stay in active tab).