fix(wasm): accept order index as a string or number in SignCancelOrder and SignModifyOrder

[lowenhere]

Problem

• WASM SignCancelOrder and SignModifyOrder read the order index via int64(args[1].Int())
• Any order index above Number.MAX_SAFE_INTEGER (2^53 - 1) is not guaranteed to be represented exactly as a JS number, so .Int() can carry a different value into the tx, which then gets signed with no error.
• This is reachable in normal use - server order indices span [2^48, 2^60-1] based on:

  lighter-go/types/txtypes/constants.go

  Lines 211 to 212 in c26ac34

  	MinOrderIndex int64 = MaxClientOrderIndex + 1
  	MaxOrderIndex int64 = (1 << 60) - 1

Fix

• Accept the index as a string or number in both functions: parse strings with strconv.ParseInt (full int64), fall back to .Int() for numbers

Validation

• GOOS=js GOARCH=wasm go build ./wasm/ and go vet pass