Fix table header alignment by removing prompt from checkbox column

The '> ' prefix was being added to the checkbox header column,
causing misalignment with the data rows.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

assets/css/dashboard.css

@@ -1463,6 +1463,11 @@ th::before {
     margin-right: 4px;
 }
 
+/* Remove prompt from checkbox header column */
+th:has(input[type="checkbox"])::before {
+    content: none;
+}
+
 tr:hover {
     background-color: rgba(0, 255, 65, 0.08);
     box-shadow: inset 0 0 20px rgba(0, 255, 65, 0.1);