feat: Alpaca US stock trader integration (Tasks 8-11)

- trader/alpaca/: Full Trader interface implementation for Alpaca
  - Paper/Live trading support (market orders, stop/limit)
  - GetBalance, GetPositions, GetMarketPrice via Alpaca API
  - Fractional share support (4 decimal places)
  - Commission-free trading

- Agent trade routing: stock symbols (AAPL, TSLA) → Alpaca
  - isStockSymbol() heuristic to distinguish crypto vs stock
  - execute_trade tool updated for stock buy/sell semantics
  - get_market_price works for both crypto and stocks

- TraderManager + API: Alpaca registered as exchange type
  - Factory case in auto_trader.go
  - Config mapping in trader_manager.go
  - Temp trader creation in handler_trader.go for balance query

- Frontend: PositionsPanel distinguishes stock vs crypto
  - US flag emoji for stock positions
  - Dollar prefix for stock PnL/prices
  - 'Shares' label instead of 'Qty' for stocks

- System prompts updated for stock trading capabilities

api/handler_trader.go

@@ -10,6 +10,7 @@ import (
 	"nofx/safe"
 	"nofx/store"
 	"nofx/trader"
+	alpacatrader "nofx/trader/alpaca"
 	"nofx/trader/aster"
 	"nofx/trader/binance"
 	"nofx/trader/bitget"
@@ -217,6 +218,12 @@ func (s *Server) handleCreateTrader(c *gin.Context) {
 			} else {
 				createErr = fmt.Errorf("Lighter requires wallet address and API Key private key")
 			}
+		case "alpaca":
+			tempTrader = alpacatrader.NewAlpacaTrader(
+				string(exchangeCfg.APIKey),
+				string(exchangeCfg.SecretKey),
+				exchangeCfg.Testnet,
+			)
 		default:
 			logger.Infof("⚠️ Unsupported exchange type: %s, using user input for initial balance", exchangeCfg.ExchangeType)
 		}