fix: backdrop now actually hidden, no longer covers the page

The #editor-backdrop rule set display:flex, which has higher specificity
than the UA style for the hidden attribute - so the modal overlay was
permanently visible on load and blocked the whole UI underneath (user-
reported: "the new record field hides everything"). Adding
#editor-backdrop[hidden] { display:none } makes the attribute win again;
opening/closing the editor still uses flex centering.

🤖 Generated with Codebuff
Co-Authored-By: Codebuff <noreply@codebuff.com>

backend/static/style.css

@@ -108,6 +108,10 @@ td:last-child, td button { white-space: nowrap; }
 td button + button { margin-left: 0.4rem; }
 
 /* modal */
+/* NOTE: keep `hidden` winning over the flex display below, otherwise the
+   backdrop is permanently visible and blocks everything underneath. */
+#editor-backdrop[hidden] { display: none; }
+
 #editor-backdrop {
   position: fixed;
   inset: 0;