style: add global user-select:none and PDF preview popup styles

app.css: Disable text/image selection globally; re-enable for
input/textarea/[contenteditable] so typing and pasting still work.

sender-page.css: Add .sender-preview-popup, .sender-preview-viewer-wrap
and .sender-preview-pdf-viewer rules. DxPdfViewer requires an explicit
pixel-height parent — percentage heights resolve to 0 inside popup body
flex containers and cause DevExpress NullReferenceExceptions.

EnvelopeGenerator.Server/EnvelopeGenerator.Server/wwwroot/css/app.css

@@ -1,5 +1,17 @@
 @import url('open-iconic/font/css/open-iconic-bootstrap.min.css');
 
+/* Disable text/image selection globally */
+* {
+    user-select: none;
+    -webkit-user-select: none;
+}
+
+/* Re-enable for inputs where typing/pasting must work */
+input, textarea, [contenteditable] {
+    user-select: text;
+    -webkit-user-select: text;
+}
+
 html, body {
     font-family: 'Helvetica Neue', Helvetica, Arial, sans-serif;
     padding: 0;

EnvelopeGenerator.Server/EnvelopeGenerator.Server/wwwroot/css/sender-page.css

@@ -479,3 +479,37 @@
     border-color: rgba(239, 68, 68, 0.3);
     color: #fca5a5;
 }
+
+/* ── PDF Preview Popup ─────────────────────────────────────────────────── */
+
+/*
+ * DxPdfViewer needs an explicit pixel-height parent — percentage heights
+ * resolve to 0 inside popup body flex containers and cause DevExpress
+ * ProcessDocumentChangedAsync NullReferenceExceptions.
+ *
+ * Strategy:
+ *   - Popup:      80vw × 90vh
+ *   - Body:       fills remaining space between header (~52px) and footer (~58px)
+ *   - Wrapper:    flex column, explicit height = calc(90vh - 52px - 58px - 2px)
+ *   - DxPdfViewer: flex:1, overflow handled internally by DevExpress (scroll works)
+ */
+.sender-preview-popup .dxbl-popup-body {
+    padding: 0 !important;
+    overflow: hidden;
+}
+
+.sender-preview-viewer-wrap {
+    display: flex;
+    flex-direction: column;
+    width: 100%;
+    height: calc(90vh - 114px); /* 52px header + 58px footer + 4px borders */
+    min-height: 480px;
+    overflow: hidden;            /* inner DxPdfViewer manages its own scroll */
+}
+
+.sender-preview-pdf-viewer {
+    flex: 1 1 auto;
+    width: 100%;
+    height: 100%;
+    overflow: auto;
+}