fix(webui): re-check focus before stealing it in DetailNavigationSheet

Stabilizes a flaky test ("does not yank focus onto a listbox option when
filteredItems shrinks"): the rAF that moves roving focus now re-checks
document.activeElement before calling focus(), so a focus bounce between
the effect's gate check and the next frame no longer steals focus.

Provisional: the race only reproduces under JSDOM focus simulation (not in
real browsers), so the proper fix likely belongs in the test layer — to be
revisited separately.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/src/components/shared/detail-navigation/detail-navigation-sheet.tsx

@@ -173,6 +173,11 @@ export function DetailNavigationSheet<T extends { id: string }>({
         }
 
         const id = requestAnimationFrame(() => {
+            // Focus may have returned to the search input since the effect's gate check.
+            if (document.activeElement === searchInputRef.current) {
+                return;
+            }
+
             const node = buttonRefs.current.get(focusedId);
 
             if (!node) {