fix(mobile-app): 点击"全部"时扣除手续费计算最大可转金额

- FeeConfig 新增 calculateMaxAmount 方法计算扣除手续费后的最大金额
- 修改提取页面 _setMaxAmount，点击"全部"时显示扣费后金额
- 固定费率：maxAmount = balance - fixedFee
- 百分比费率：maxAmount = balance / (1 + rate)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/mobile-app/lib/core/services/wallet_service.dart

@@ -181,6 +181,36 @@ class FeeConfig {
     }
   }
 
+  /// 计算给定余额下的最大可转金额
+  /// 因为手续费由发送方承担，所以 总花费 = 转账金额 + 手续费
+  /// 返回能够使 转账金额 + 手续费 <= balance 的最大转账金额
+  double calculateMaxAmount(double balance) {
+    if (balance <= 0) return 0;
+
+    if (feeType == FeeType.fixed) {
+      // 固定费率: maxAmount = balance - fixedFee
+      final maxAmount = balance - feeValue;
+      return maxAmount > 0 ? maxAmount : 0;
+    } else {
+      // 百分比费率: 总花费 = amount * (1 + rate)
+      // 所以 maxAmount = balance / (1 + rate)
+      final maxAmount = balance / (1 + feeValue);
+
+      // 检查最小/最大手续费限制
+      final fee = calculateFee(maxAmount);
+      final actualTotal = maxAmount + fee;
+
+      // 如果由于 minFee 导致超出余额，需要重新计算
+      if (actualTotal > balance && minFee > 0) {
+        // 使用 minFee 计算: maxAmount = balance - minFee
+        final adjustedMax = balance - minFee;
+        return adjustedMax > 0 ? adjustedMax : 0;
+      }
+
+      return maxAmount > 0 ? maxAmount : 0;
+    }
+  }
+
   /// 获取费率描述
   String get description {
     if (feeType == FeeType.fixed) {

frontend/mobile-app/lib/features/withdraw/presentation/pages/withdraw_usdt_page.dart

@@ -167,9 +167,18 @@ class _WithdrawUsdtPageState extends ConsumerState<WithdrawUsdtPage> {
     });
   }
 
-  /// 设置全部金额
+  /// 设置全部金额（扣除手续费后的最大可转金额）
   void _setMaxAmount() {
-    _amountController.text = _usdtBalance.toStringAsFixed(2);
+    double maxAmount;
+    if (_feeConfig != null) {
+      maxAmount = _feeConfig!.calculateMaxAmount(_usdtBalance);
+    } else {
+      // 默认固定 2 绿积分手续费
+      maxAmount = _usdtBalance - 2;
+    }
+    // 确保金额不为负
+    if (maxAmount < 0) maxAmount = 0;
+    _amountController.text = maxAmount.toStringAsFixed(2);
     setState(() {});
   }