revert(review-desk): restore replaceState for URL sync

Task 16 changed history.replaceState to pushState, described as
'back/forward restoration'. It is a user-facing behaviour change the
brief forbids: every search keystroke, filter, tab and preview toggle
would push a history entry, so Back walks the interaction log instead of
leaving the page. main's popstate handler already existed and works the
same either way.

skills-review/app.js

@@ -278,7 +278,7 @@ function syncUrl() {
   else url.searchParams.delete('render');
   if (state.diff) url.searchParams.set('compare', 'diff');
   else url.searchParams.delete('compare');
-  history.pushState({}, '', url);
+  history.replaceState({}, '', url);
 }
 function selectFromUrl() {
   const params = new URLSearchParams(window.location.search);