Add a global settings page

Currently, the editor does not have global settings of any sort.
Some example settings could be:

- A default location where files are saved
- Global default font size (the current default, at least on MacOS is quite large)
- Whether the last opened file should be opened on startup
- Use the dark/light theme by default (I know this is not implemented yet)

No file, test, or entry point is named. Start by surveying how the desktop editor currently handles saved-file locations, font defaults, startup restoration, and themes, then clarify which settings are in scope. Done should mean the selected global settings work consistently; theme support is explicitly noted as not yet implemented.

Written by the indexing model from the issue text.