🐛 Guard event.key before toLowerCase in search keydown handler

The global keydown handler calls event.key.toLowerCase() without checking
that event.key is defined. Some keydown events carry no key property
(browser autofill, IME composition, some mobile virtual keyboards), which
throws TypeError: Cannot read properties of undefined (reading
'toLowerCase') and breaks the handler for that keystroke.

Add a truthiness guard before calling toLowerCase(), matching the style
already used for the other event.key comparisons in the same handler.
No behavior change for real Cmd/Ctrl+K presses.

assets/js/search.js

@@ -42,7 +42,7 @@ document.addEventListener("keydown", function (event) {
   }
 
   // Cmd+K (macOS) / Ctrl+K to toggle search wrapper
-  if (event.key.toLowerCase() == "k" && (event.metaKey || event.ctrlKey)) {
+  if (event.key && event.key.toLowerCase() == "k" && (event.metaKey || event.ctrlKey)) {
     event.preventDefault();
     if (searchVisible) {
       hideSearch();