refactor(sender): overhaul PDF preview popup layout and styling

Replace inline styles and magic-number height calculations with a proper
BEM CSS class system. Attach HeaderCssClass and BodyCssClass to DxPopup
so DevExpress DOM targets can be styled reliably without deep selector
hacks.

Layout changes:
- Remove hardcoded calc(90vh - 114px) height from viewer wrapper; use
  flex layout so the body fills available space naturally
- Change overflow on DxPdfViewer wrapper from hidden to auto to allow
  horizontal scrolling at high zoom levels
- Center PDF pages horizontally inside dxbrv-document-surface via flex

New CSS classes (BEM):
- .sender-preview-popup           root shell, accent top border
- .sender-preview-popup__header   styled header via HeaderCssClass
- .sender-preview-popup__body     flex column body via BodyCssClass
- .sender-preview-viewer-wrap     padding wrapper for the PDF viewer
- .sender-preview-pdf-viewer      viewer shell with rounded corners / shadow
- .sender-preview-state-wrap      centered container for loading/error states
- .sender-preview-error-card      error message card with left accent border
- .sender-preview-error-icon      circular icon wrapper
- .sender-preview-error-title/msg typography helpers

Theme support:
- Purple, Blazing Dark variants via data-sf-theme attribute overrides

Custom scrollbars:
- WebKit scrollbar styling scoped to .sender-preview-popup__body

Inline style cleanup in EnvelopeSenderPage.razor:
- Replace all ad-hoc style= attributes with the new CSS classes above

EnvelopeGenerator.Server/EnvelopeGenerator.Server/Components/Pages/EnvelopeSenderPage.razor

@@ -441,6 +441,8 @@
          CloseOnOutsideClick="true"
          ShowFooter="false"
          CssClass="sender-preview-popup"
+         HeaderCssClass="sender-preview-popup__header"
+         BodyCssClass="sender-preview-popup__body"
          Closed="OnPreviewClosed"
          AllowResize="true"
          ApplyBackgroundShading="true"
@@ -451,7 +453,7 @@
     <BodyContentTemplate>
         @if (_previewLoading)
         {
-            <div style="display:flex;justify-content:center;align-items:center;height:100%;min-height:300px;">
+            <div class="sender-preview-state-wrap">
                 <div class="text-center">
                     <div class="spinner-border mb-3" style="width:3rem;height:3rem;" role="status">
                         <span class="visually-hidden">Lädt...</span>
@@ -462,27 +464,24 @@
         }
         else if (_previewErrorMessage is not null)
         {
-            <div style="display:flex;gap:1rem;align-items:flex-start;padding:1.5rem;">
-                <div style="flex-shrink:0;width:40px;height:40px;border-radius:50%;
-                            background:#fee2e2;display:flex;align-items:center;justify-content:center;">
-                    <svg xmlns="http://www.w3.org/2000/svg" width="20" height="20" fill="#dc2626" viewBox="0 0 16 16">
-                        <path d="M8 15A7 7 0 1 1 8 1a7 7 0 0 1 0 14zm0 1A8 8 0 1 0 8 0a8 8 0 0 0 0 16z" />
-                        <path d="M7.002 11a1 1 0 1 1 2 0 1 1 0 0 1-2 0zM7.1 4.995a.905.905 0 1 1 1.8 0l-.35 3.507a.552.552 0 0 1-1.1 0L7.1 4.995z" />
-                    </svg>
-                </div>
-                <div>
-                    <p style="margin:0 0 0.4rem;font-weight:600;color:#1f2937;font-size:0.95rem;">
-                        Dokument konnte nicht geladen werden
-                    </p>
-                    <p style="margin:0;color:#6b7280;font-size:0.85rem;line-height:1.5;word-break:break-word;">
-                        @_previewErrorMessage
-                    </p>
+            <div class="sender-preview-state-wrap">
+                <div class="sender-preview-error-card">
+                    <div class="sender-preview-error-icon">
+                        <svg xmlns="http://www.w3.org/2000/svg" width="20" height="20" fill="#dc2626" viewBox="0 0 16 16">
+                            <path d="M8 15A7 7 0 1 1 8 1a7 7 0 0 1 0 14zm0 1A8 8 0 1 0 8 0a8 8 0 0 0 0 16z" />
+                            <path d="M7.002 11a1 1 0 1 1 2 0 1 1 0 0 1-2 0zM7.1 4.995a.905.905 0 1 1 1.8 0l-.35 3.507a.552.552 0 0 1-1.1 0L7.1 4.995z" />
+                        </svg>
+                    </div>
+                    <div>
+                        <p class="sender-preview-error-title">Dokument konnte nicht geladen werden</p>
+                        <p class="sender-preview-error-msg">@_previewErrorMessage</p>
+                    </div>
                 </div>
             </div>
         }
         else if (_previewPdfBytes is not null && _previewViewerMounted)
         {
-            <div class="sender-preview-viewer-wrap" style="padding: 0 5.5rem;">
+            <div class="sender-preview-viewer-wrap">
                 <DxPdfViewer DocumentContent="@_previewPdfBytes"
                              ZoomLevel="-2"
                              IsSinglePagePreview="true"