Bug report: Utils.modInv assumes modulus of 26

**Describe the bug**
The Utils.modInv function internally uses 26 as the modulus rather than the argument `y`. The bug is not externally visible because it's used in the Affine Decode operation, which uses an alphabet with 26 letters.

**Source code**
```
static modInv(x, y) {
x %= y;
for (let i = 1; i < y; i++) {
if ((x * i) % 26 === 1) {
return i;
}
}
}
```

**Remedy**
Replace `26` with `y`.

Better still: replace the entire function with one that uses the Extended Euclidean Algorithm to compute the modular multiplicative inverse. It will be faster than a brute-force loop for large values of `y`.

Locate Utils.modInv in the JavaScript source and inspect its use by the Affine Decode operation. Confirm how the modulus argument is intended to work, then replace the hard-coded modulus with y or evaluate the proposed Extended Euclidean Algorithm; done means modular inverses use the supplied modulus without changing Affine Decode behavior.

Written by the indexing model from the issue text.