feat(web): add step-back to replay viewer

The "⏮ Restart" button now steps back one move at a time instead of
resetting to the beginning. Re-creates the ReplayPlayer and fast-forwards
to (step_idx - 1) without rendering intermediate frames; the CSS transform
transition then animates each card back to its previous position.

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

solitaire_server/web/index.html

@@ -30,7 +30,7 @@
         <section id="board"></section>
 
         <section id="controls">
-            <button id="btn-prev" disabled>⏮ Restart</button>
+            <button id="btn-prev" disabled>◀ Back</button>
             <button id="btn-play">▶ Play</button>
             <button id="btn-step">⏭ Step</button>
             <span id="progress" class="muted">step 0 / 0</span>