Fix mobile layout: stack search input and button vertically (#240)

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

Co-authored-by: Claude <noreply@anthropic.com>

Author: simonw

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; }