feat(web): add undo button directly on the game board

Places a floating "↩ Undo" button at the bottom-right of the green felt
surface so it is visible without looking in the header. Both the board
button and the header button share the same handler; both track
undo_stack_len and disable when nothing can be undone.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

solitaire_server/web/game.html

@@ -46,6 +46,7 @@
     <main>
         <section id="board">
             <div id="card-area"></div>
+            <button id="btn-board-undo" class="board-undo" title="Undo (Z)" disabled>↩ Undo</button>
         </section>
     </main>