fix: use taker fees (BG 0.06%, HL 0.045%) + threshold 0.25%

All IOC market orders incur taker fees, not maker fees. Previous
makerFees (BG 0.02%, HL 0.015%) caused 0.10% threshold trades to
actually lose 0.11% per round trip.

Changes:
- scanner.go: makerFees → takerFees (BG 0.060, HL 0.045)
- dashboard.go: makerFees → takerFees
- types.go: makerFees → takerFees
- trader.go: makerFees → takerFees
- config.json: trade_threshold 0.1 → 0.25

config.json

@@ -3,7 +3,7 @@
   "trade_enabled": true,
   "arb_threshold": 0.03,
   "scan_interval_ms": 200,
-  "trade_threshold": 0.1,
+  "trade_threshold": 0.25,
   "trade_amount_usd": 5,
   "trade_cooldown_ms": 30000,
   "alert_cooldown_sec": 300,