fix: resolve all remaining lint errors (errcheck) across 49 files

Fix unchecked error returns on bf.Seek(), db.Exec(), QueryRow().Scan(),
pkt.Build(), logger.Sync(), and binary.Write() calls. The linter now
passes with 0 errors, build compiles, and all tests pass with -race.

common/mhfitem/mhfitem_test.go

@@ -303,7 +303,7 @@ func TestReadWarehouseEquipment_ZeroWarehouseID(t *testing.T) {
 	}
 	bf.WriteUint16(0)
 
-	bf.Seek(0, 0)
+	_, _ = bf.Seek(0, 0)
 	equipment := ReadWarehouseEquipment(bf)
 
 	if equipment.WarehouseID == 0 {