Fix Listings button functionality

- the previous and next buttons took people to /browse/PAGENUMBER rather
  than /browse/?page=PAGENUMBER, where the frontend expects the latter.
  Fixed.
- having fixed that, when we go to a new page, the listings do not
  refresh because the useEffect calling handleSearch has never been
  called; the useEffect hook now updates when pageNumber changes.

frontend/src/pages/Browse.tsx

@@ -28,7 +28,7 @@ const Browse = () => {
 
   useEffect(() => {
     handleSearch();
-  }, [debouncedSearch]);
+  }, [debouncedSearch, pageNumber]);
 
   const handleSearch = async () => {
     setListingIds(null);
@@ -90,15 +90,15 @@ const Browse = () => {
             {prevButton && (
               <button
                 className="bg-gray-300 hover:bg-gray-400 text-gray-800 font-bold py-2 px-4 rounded-l mr-auto"
-                onClick={() => navigate(`/browse/${pageNumber - 1}`)}
+                onClick={() => navigate(`/browse/?page=${pageNumber - 1}`)}
               >
                 Previous
               </button>
             )}
             {nextButton && (
               <button
                 className="bg-gray-300 hover:bg-gray-400 text-gray-800 font-bold py-2 px-4 rounded-r ml-auto"
-                onClick={() => navigate(`/browse/${pageNumber + 1}`)}
+                onClick={() => navigate(`/browse/?page=${pageNumber + 1}`)}
               >
                 Next
               </button>