Fix button alignment on book detail page when wrapping

Replace .btn + .btn { margin-left } with margin-right/margin-bottom on
.book-detail-body .btn so the "Im Browser lesen" button stays
left-aligned when the buttons wrap to a new line on narrow viewports.
Also drop the now-unused .reader-nav .btn + .btn override.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

static/styles.css

@@ -203,8 +203,8 @@ h2 {
   color: inherit;
 }
 
-.btn + .btn {
-  margin-left: 0.5rem;
+.book-detail-body .btn {
+  margin: 0 0.5rem 0.5rem 0;
 }
 
 .reader {
@@ -243,10 +243,6 @@ h2 {
   margin-top: 1.5rem;
 }
 
-.reader-nav .btn + .btn {
-  margin-left: 0;
-}
-
 .reader-nav-next {
   margin-left: auto;
 }