Add option to switch between light and dark theme

[ondrej33]

Currently, the app's mode (light or dark) is chosen automatically based on the device settings. It would be useful to give users the option to switch between the two modes.

This could initially be a new button in the main menu. Later, if we have more such choices (such as an optional developer mode or base app font size), we can extract all of these options elsewhere.

[ondrej33]

After some searching, I believe that Tauri 1 does not support dynamic theme switching, but it was added in Tauri 2 (see https://v2.tauri.app/reference/javascript/api/namespaceapp/#settheme).

So, this seems like a simple thing to do, but only if/after we migrate to Tauri 2. We could use some plugin/workaround till then, but I don't think it is a major problem for now.