Need abiltity to save user settings

#1833 re secrets and API keys is one use case, but there will be more:
- keybindings
- preferred pane layout
- text wrap setting
- etc?

this will require some back-end work to allow settings to be saved to the server. this part should aim to be pretty generic; maybe one generic table for storing json, and a high-level client side API.

front end stuff, OTOH, might need to be pretty specific. there could be one "settings" modal, but e.g. a keybindings setting page might have different needs than a secrets settings page. or perhaps they could all just be subcomponents on one modal page initially. but in any case, each kind of setting will require it's own little bit of front end work.

Start by reviewing issue #1833 and tracing the existing client/server settings or persistence architecture; the issue does not name files, tests, or entry points. Done should include a generic server-side way to save user settings and a high-level client API, with the frontend treatment for each setting type decided.

Written by the indexing model from the issue text.