fix(stamp): cast before min initial balance multiply

[0xCardiE]

Summary

• Cast minimumValidityBlocks and lastPrice to uint256 before multiplying in minimumInitialBalancePerChunk().
• Prevents silent uint64 overflow when computing the minimum required per-chunk balance.

Test plan

• Run existing PostageStamp unit tests
• Verify minimumInitialBalancePerChunk() matches uint256(minimumValidityBlocks) * uint256(lastPrice) at high lastPrice values
• Confirm batch create/topup/expiry checks still enforce the minimum balance correctly