Serve practice mode from the main server, add mode switch, fix touch

Practice mode is now at /practice on server.py instead of a separate
server, with a button in the bottom-right corner of each page to the
other mode. Touch: no double-tap zoom, no sticky hover after a tap, no
long-press selection on the board, and a pressed look on tapped squares.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

static/practice.html

@@ -42,6 +42,7 @@
       <p id="meta" class="meta"></p>
     </div>
   </main>
+  <a class="mode-switch" href="/">Multiplayer</a>
   <script src="/static/board.js"></script>
   <script src="/static/practice.js"></script>
 </body>