Fix preview thumbnails being squished

https://pixwhile.zangetsu.kaizoku.cyou/artworks/108749543?preview=1

AFAIK, this does not interfere with 42da969ac5
and 5afccd3616

routes/css.cpp

@@ -30,6 +30,7 @@ static const constexpr char css[] = R"EOF(
     img {
         max-width: 100%;
         height: auto; /* https://stackoverflow.com/a/17183996 */
+        object-fit: cover;
     }
     .center {
         text-align: center;
@@ -112,7 +113,6 @@ static const constexpr char css[] = R"EOF(
     .user-cover {
         width: 100%;
         height: 50vh;
-        object-fit: cover;
         margin-bottom: 1em;
     }