fix(trading): 卖出金额超过可用积分股时提示并禁用确认按钮

- 新增 _getSellValidationError 校验方法
- 输入金额反算的积分股数量 > 可用余额时显示红色提示
- 校验失败时禁用"确认交易"按钮（灰色）

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

frontend/mining-app/lib/presentation/pages/trading/trading_page.dart

@@ -645,10 +645,19 @@ class _TradingPageState extends ConsumerState<TradingPage> {
               _selectedTab == 0 ? availableCash : null,
               currentPrice,
             ),
-            // 卖出时显示金额输入框（双向联动）
+            // 卖出时显示金额输入框（双向联动）+ 余额校验
             if (_selectedTab == 1) ...[
               const SizedBox(height: 16),
               _buildAmountInput(tradingShareBalance, currentPrice),
+              // 积分股不足提示
+              if (_getSellValidationError(tradingShareBalance) != null)
+                Padding(
+                  padding: const EdgeInsets.only(top: 8),
+                  child: Text(
+                    _getSellValidationError(tradingShareBalance)!,
+                    style: const TextStyle(fontSize: 12, color: _red),
+                  ),
+                ),
             ],
             const SizedBox(height: 16),
             // 买入时显示预计支出（卖出时金额输入框已替代预计获得）
@@ -701,14 +710,17 @@ class _TradingPageState extends ConsumerState<TradingPage> {
                 ),
               ),
             const SizedBox(height: 24),
-            // 提交按钮
-            SizedBox(
+            // 提交按钮（卖出时校验积分股余额）
+            Builder(builder: (context) {
+              final sellError = _selectedTab == 1 ? _getSellValidationError(tradingShareBalance) : null;
+              final isDisabled = sellError != null;
+              return SizedBox(
               width: double.infinity,
               height: 48,
               child: ElevatedButton(
-                onPressed: _handleTrade,
+                onPressed: isDisabled ? null : _handleTrade,
                 style: ElevatedButton.styleFrom(
-                  backgroundColor: _orange,
+                  backgroundColor: isDisabled ? Colors.grey : _orange,
                   shape: RoundedRectangleBorder(
                     borderRadius: BorderRadius.circular(12),
                   ),
@@ -722,7 +734,8 @@ class _TradingPageState extends ConsumerState<TradingPage> {
                   ),
                 ),
               ),
-            ),
+            );
+            }),
           ],
         ],
       ),
@@ -957,6 +970,17 @@ class _TradingPageState extends ConsumerState<TradingPage> {
     );
   }
 
+  /// 卖出校验：返回错误信息，null 表示通过
+  String? _getSellValidationError(String tradingShareBalance) {
+    final quantity = double.tryParse(_quantityController.text) ?? 0;
+    final available = double.tryParse(tradingShareBalance) ?? 0;
+    if (quantity <= 0) return null; // 空输入不提示
+    if (quantity > available) {
+      return '交易账户积分股不足，需要 ${quantity.toStringAsFixed(4)}，可用 ${formatAmount(tradingShareBalance)}';
+    }
+    return null;
+  }
+
   /// 获取卖出系数：(1 + burnMultiplier) × price × 0.9
   double _getSellFactor() {
     final price = double.tryParse(_priceController.text) ?? 0;