Fix mobile layout: stack search input and button vertically

The search row's flex layout caused the Search button to be cut off
on narrow screens. Added mobile breakpoint rules to stack the input
and button vertically at <=600px width.

https://claude.ai/code/session_01DteQHQeBGUnMUZrmiN7vDh

Author: claude

unicode-binary-search.html

@@ -270,6 +270,8 @@
 
   @media (max-width: 600px) {
     .container { padding: 1rem 0.75rem; }
+    .search-row { flex-direction: column; }
+    .search-btn { width: 100%; }
     .result-card.visible { flex-direction: column; text-align: center; }
     .result-char { font-size: 72px; min-width: auto; }
     .log-table { font-size: 0.7rem; }