feat: Chess.com-style smart tap-to-move controls for backgammon

Smart behaviors:
- Auto-roll dice at start of each turn
- If only one move exists for a piece, execute immediately on tap
- If only one complete sequence possible, auto-execute entire turn
- Auto-end turn when no moves remain
- Movable pieces highlighted with golden glow (tap targets)
- Tap piece → valid destinations highlighted → tap destination → done
- Undo button for unlimited undo before turn ends

UX flow:
1. Dice auto-roll
2. Movable sources highlighted
3. Tap source → if single destination, auto-move
4. If multiple destinations → show targets → tap one
5. Repeat until dice exhausted
6. Turn auto-ends

Minimal taps, zero rules knowledge needed, fast animations.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

pages/backgammon-game.php

@@ -30,10 +30,11 @@
 
   <!-- Side panel -->
   <div class="bg-side-panel">
-    <div id="bg-turn" class="bg-turn-indicator"></div>
     <div id="bg-dice-container" class="bg-dice-area"></div>
+    <div id="bg-status" class="bg-status"></div>
     <div class="bg-controls">
       <button id="bg-roll-btn" class="bg-roll-btn">ارمِ النرد</button>
+      <button id="bg-undo-btn" class="bg-pass-btn" style="display:none;">تراجع</button>
     </div>
     <div id="bg-log" class="bg-log"></div>
   </div>

public/css/backgammon.css

@@ -176,12 +176,14 @@
     border-radius: 4px;
 }
 
-.bg-point--clickable {
+.bg-point--source {
     cursor: pointer;
 }
 
-.bg-point--clickable .bg-checker:last-child {
-    box-shadow: 0 0 0 2px rgba(255,200,50,0.4), 0 2px 4px rgba(0,0,0,0.3);
+.bg-point--source .bg-checker:last-child,
+.bg-point--source .bg-checkers-stack .bg-checker:last-child {
+    box-shadow: 0 0 0 3px rgba(255,200,50,0.5), 0 0 10px rgba(255,200,50,0.3);
+    transform: scale(1.05);
 }
 
 .bg-point--hover-preview {