refactor(phase10): manage staging buttons via dedicated lifecycle methods

- Extract button creation and destruction into `_createStagingButtons` and `_destroyStagingButtons`.
- Ensure buttons are properly destroyed when staging is cleared, submitted, or when turn conditions change.
- Prevent memory leaks and stale UI state by centralizing button lifecycle management.
- Add piece click sound effect in Backgammon on landing.

public/src/games/backgammon/BackgammonGame.js

@@ -802,7 +802,10 @@ export default class BackgammonGame extends Phaser.Scene {
         container.x = to.x;
         container.y = to.y;
         // Squash-and-stretch on landing
-        this.tweens.add({ targets: container, scaleX: 1.3, scaleY: 0.7, duration: 60, yoyo: true, ease: 'Quad.easeOut', onComplete });
+        this.tweens.add({ targets: container, scaleX: 1.3, scaleY: 0.7, duration: 60, yoyo: true, ease: 'Quad.easeOut', onComplete: () => {
+          playSound(this, SFX.PIECE_CLICK);
+          onComplete();
+        }});
       },
     });
   }

public/src/games/phase10/Phase10Game.js

@@ -152,6 +152,8 @@ export default class Phase10Game extends Phaser.Scene {
     this.dragState = null;
     this.potentialDrag = null;
     this.stagingGroups = null;
+    this.submitBtn = null;
+    this.clearBtn  = null;
   }
 
   create() {
@@ -761,16 +763,6 @@ export default class Phase10Game extends Phaser.Scene {
       cx += gw + GROUP_GAP;
     }
 
-    const btnX = panelX + panelW + 160;
-    const submitBtn = new Button(this, btnX, laidStart.y - 18, 'Submit Phase', () => this.submitStagingLaydown(), {
-      width: 200, height: 44, fontSize: 18,
-    }).setDepth(D.ui);
-    this.transientObjs.push(submitBtn);
-
-    const clearBtn = new Button(this, btnX, laidStart.y + 34, 'Clear', () => this.clearStagingLaydown(), {
-      variant: 'ghost', width: 200, height: 34, fontSize: 14,
-    }).setDepth(D.ui);
-    this.transientObjs.push(clearBtn);
   }
 
   submitStagingLaydown() {
@@ -798,6 +790,7 @@ export default class Phase10Game extends Phaser.Scene {
       return;
     }
     this.stagingGroups = null;
+    this._destroyStagingButtons();
     this.gs = next;
     this.selectedHandIdx = null;
     this.clearHighlights();
@@ -954,6 +947,7 @@ export default class Phase10Game extends Phaser.Scene {
     this.matchOver = false;
     if (this.dragState) this.endCardDragImmediate();
     this.stagingGroups = null;
+    this._destroyStagingButtons();
     this.clearAllCardObjs();
     this.clearHighlights();
     this.selectedHandIdx = null;
@@ -1241,6 +1235,7 @@ export default class Phase10Game extends Phaser.Scene {
     const eligible = this.isLocalTurn() && this.gs.drawnThisTurn && !player.laidDown;
     if (!eligible) {
       this.stagingGroups = null;
+      this._destroyStagingButtons();
       return;
     }
     if (!this.stagingGroups) {
@@ -1251,9 +1246,34 @@ export default class Phase10Game extends Phaser.Scene {
         count: g.count,
         slots: new Array(g.count).fill(null),
       }));
+      this._createStagingButtons();
     }
   }
 
+  _createStagingButtons() {
+    this._destroyStagingButtons();
+    const { laidStart } = slotLayout('bottom');
+    let totalW = 0;
+    for (let i = 0; i < this.stagingGroups.length; i++) {
+      if (i > 0) totalW += GROUP_GAP;
+      totalW += this.stagingGroups[i].count * (LAIDOWN_CARD_W * 0.75) + (LAIDOWN_CARD_W * 0.25);
+    }
+    const panelX = laidStart.x - 14;
+    const panelW  = totalW + 28;
+    const btnX    = panelX + panelW + 160;
+    this.submitBtn = new Button(this, btnX, laidStart.y - 18, 'Submit Phase', () => this.submitStagingLaydown(), {
+      width: 200, height: 44, fontSize: 18,
+    }).setDepth(D.ui);
+    this.clearBtn = new Button(this, btnX, laidStart.y + 34, 'Clear', () => this.clearStagingLaydown(), {
+      variant: 'ghost', width: 200, height: 34, fontSize: 14,
+    }).setDepth(D.ui);
+  }
+
+  _destroyStagingButtons() {
+    if (this.submitBtn) { this.submitBtn.destroy(); this.submitBtn = null; }
+    if (this.clearBtn)  { this.clearBtn.destroy();  this.clearBtn  = null; }
+  }
+
   // ── Local input ─────────────────────────────────────────────────────────
 
   isLocalTurn() {