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

.gitignore

@@ -1,5 +1,6 @@
 # Binaries
 exchange-monitor
+main
 
 # Logs
 *.log