Implement newsletter preview copy functionality and UI enhancements

- Add a new preview banner to indicate the live newsletter display.
- Introduce `copyFromPreviewFrame` function to enable copying the newsletter preview directly.
- Update copy status messages for clarity and improved user guidance.
- Revise button labels in the dashboard for better usability.
- Modify CSS for the newsletter preview frame and add styles for the new preview banner.

app/static/css/main.css

@@ -487,13 +487,27 @@ tbody tr:hover { background: #FAFAFA; }
 .copy-status.success { color: var(--tk-success); }
 .copy-status.error { color: var(--tk-error); }
 
+.preview-banner {
+    margin-top: 0.5rem;
+    padding: 0.5rem 0.85rem;
+    background: var(--tk-orange-light);
+    border: 1px solid #F5D5BC;
+    border-bottom: none;
+    border-radius: var(--radius) var(--radius) 0 0;
+    font-size: 0.82rem;
+    font-weight: 600;
+    color: var(--tk-orange-dark);
+    text-align: center;
+}
+
 .newsletter-preview-frame {
     width: 100%;
     min-height: 640px;
     border: 1px solid var(--tk-border);
-    border-radius: var(--radius);
-    background: var(--tk-gray-light);
+    border-radius: 0 0 var(--radius) var(--radius);
+    background: #FFFFFF;
     box-shadow: var(--shadow-md);
+    display: block;
 }
 
 .textarea-actions {