bug: EVMChainID uint64 silently overflows to negative int64 in NewBackend

[Aboudjem]

Bug Description

In rpc/backend/backend.go, NewBackend() converts EVMChainID (uint64) to int64 unsafely:

EvmChainID: big.NewInt(int64(appConf.EVM.EVMChainID)),

When EVMChainID > math.MaxInt64 (9223372036854775807), the uint64 wraps to a negative int64, creating a negative big.Int chain ID.

Impact

• All EIP-155 transaction signature verification fails with a negative chain ID
• Potential replay attacks if different chains end up with colliding (wrapped) chain IDs
• The config field EVMChainID is typed as uint64, so values > MaxInt64 are valid configuration

Proposed Fix

Replace big.NewInt(int64(...)) with the safe SetUint64 method:

EvmChainID: new(big.Int).SetUint64(appConf.EVM.EVMChainID),

One-line fix, zero behavioral change for chain IDs within int64 range.

[linear]

EVM-439

[swift1337]

Closing for being a low-effort AI submission.