feat: add USDT warning for Aster exchange configuration

Added warning message to inform users that Aster only tracks USDT balance, preventing P&L calculation errors from asset price fluctuations.

Why this is important:
- Aster trader only tracks USDT balance (aster_trader.go:453)
- If users use BNB/ETH as margin, price fluctuations will cause:
  * Initial balance becomes inaccurate
  * P&L statistics will be wrong
  * Example: 10 BNB @ $100 = $1000, if BNB drops to $90, real equity is $900 but system still shows $1000

Changes:
- Added asterUsdtWarning translation in both EN and ZH
- Added red warning box below Aster private key field
- Clear message: "Please use USDT as margin currency"

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

Co-Authored-By: tinkle-community <tinklefund@gmail.com>

web/src/components/AITradersPage.tsx

@@ -1899,6 +1899,26 @@ function ExchangeConfigModal({
                       required
                     />
                   </div>
+
+                  {/* USDT 警告提示 */}
+                  <div
+                    className="p-4 rounded"
+                    style={{
+                      background: 'rgba(246, 70, 93, 0.1)',
+                      border: '1px solid rgba(246, 70, 93, 0.3)',
+                    }}
+                  >
+                    <div
+                      className="text-sm font-semibold mb-2 flex items-center gap-2"
+                      style={{ color: '#F6465D' }}
+                    >
+                      <AlertTriangle className="w-4 h-4" />
+                      {t('securityWarning', language)}
+                    </div>
+                    <div className="text-xs" style={{ color: '#848E9C' }}>
+                      {t('asterUsdtWarning', language)}
+                    </div>
+                  </div>
                 </>
               )}