Rewrite toInt() to fail on invalid input

`toInt()` has a variant that sets a success indicator. That's should always be used when validating external input (even stored in a settings ini file, really).

_Originally posted by @pljones in https://github.com/jamulussoftware/jamulus/issues/3772#issuecomment-5257629528_

Search the C codebase for toInt() and its variant that sets a success indicator. Trace callers that validate external input, including settings loaded from an ini file, and check how invalid values are handled. Done means the relevant validation paths consistently detect invalid input instead of silently accepting it.

Written by the indexing model from the issue text.