Autosave Manifest

The 'Save to manifest.json' button is easily missed.

> Ste — 29/11/2024 13:44
> would we rather automatically save to the manifest or have an unsaved indicator.. i find myself forgetting to save too often. i think saving an invalid manifest shouldn't be too big of a deal either, we're validating it mod loader side anyway.

Start by locating the “Save to manifest.json” button and the code that handles manifest saving and validation. Clarify whether the desired behavior is automatic saving, an unsaved indicator, or both, then define how invalid manifests should be handled. Done means the chosen behavior is implemented and its save state is clear to users.

Written by the indexing model from the issue text.