feat(frontend): 优化加密价差监控页面体验

- 刷新页面时保留当前选择的策略（localStorage 缓存）
- 移动端下单弹窗改为 BottomSheet 样式，更紧凑
- 买入按钮实时显示当前价格，如 "买入 Up (0.50)"
- 周期切换时自动关闭弹窗并提示用户

Made-with: Cursor

frontend/src/locales/en/common.json

@@ -1685,7 +1685,8 @@
       "maxSize": "Max",
       "invalidPriceOrBalance": "Invalid price or balance",
       "insufficientBalanceForMax": "Insufficient balance for max size",
-      "maxSizeUpdated": "Updated to max size"
+      "maxSizeUpdated": "Updated to max size",
+      "periodChanged": "Period has changed, popup closed"
     }
   }
 }

frontend/src/locales/zh-CN/common.json

@@ -1685,7 +1685,8 @@
       "maxSize": "最大",
       "invalidPriceOrBalance": "价格或余额无效",
       "insufficientBalanceForMax": "余额不足以购买最大数量",
-      "maxSizeUpdated": "已更新为最大数量"
+      "maxSizeUpdated": "已更新为最大数量",
+      "periodChanged": "周期已切换，弹窗已关闭"
     }
   }
 }

frontend/src/locales/zh-TW/common.json

@@ -1685,7 +1685,8 @@
       "maxSize": "最大",
       "invalidPriceOrBalance": "價格或餘額無效",
       "insufficientBalanceForMax": "餘額不足以購買最大數量",
-      "maxSizeUpdated": "已更新為最大數量"
+      "maxSizeUpdated": "已更新為最大數量",
+      "periodChanged": "週期已切換，彈窗已關閉"
     }
   }
 }