feat: remove toml config file only use our QSettings based format - #8849

Right now, this feature is still valuable for running and debugging the Core (client/server) directly without the GUI.

If we remove it before we have a Qt settings replacement (where the Core reads directly from Qt settings rather than the CLI opts/args), we make Core development and testing harder in the short term. Implementing Qt settings first avoids that problem and gives us an immediate quality-of-life improvement.

We’ll need Qt settings eventually anyway, since passing Core options via the CLI is awkward/clumsy. A Qt settings file is cleaner, easier to maintain, and more aligned with how we’ll want to manage configuration long-term.

I’m happy to help build the replacement if you don’t want to take it on alone. But whichever way we do it, sequencing it before removing the TOML config parser means we avoid breaking current workflows while still moving toward the end goal.