feat: real trading mode, auto-stop after 5 trades, HL testnet support

- config.json: test_mode=false, ready for sim/testnet trading
- trader.go: auto-stop after 5 real trades, exchange response logging,
  Stop()/Start() API, shuttingDown flag for graceful stop
- dashboard.go: POST /api/stop + POST /api/start endpoints,
  trading status in SSE stats
- exchange/hyperliquid.go: switch HL WS to testnet endpoint
- exchange/hyperliquid_trade.go: switch REST to testnet endpoint,
  support base64 + 32-byte EVM private keys
- main.go: listen on trader.StopCh (graceful, no process exit)
- scanner.go: trim TrackedCoins to only 6 core coins (DOGE/LINK/ONDO/OP/WIF/ARB)
- .gitignore: ignore main binary

main.go

@@ -152,6 +152,10 @@ func main() {
 			cancel() // B#1: cancel context to stop all WS goroutines
 			runLoop = false
 
+		case <-trader.StopCh:
+			log.Println("[Monitor] 5 real trades completed — trading stopped. System still running (dashboard active)")
+			log.Println("[Monitor] Use POST /api/start to resume trading, POST /api/stop to stop manually")
+
 		case <-statusTick.C:
 			snap := store.GetAll()
 			count := 0