Detect System Color Theme

[NandeMD]

This PR adds two things:

• On the first startup, (if there is no $HOME/.config/Psst/config.json file), app tries to detect the system theme, falls back to Theme::Light on error or unspecified theme.
• A new button on the Preferences -> General Tab -> Theme. When pressed, it tries to detect the system theme, falls back to Theme::Light on error or unspecified theme:

Screencast_20250725_143112.webm

[jacksongoode]

Super cool thank you! Did you see #162 and #160 and #150 ? If so that's great, I haven't yet looked at the code but I remember there was an issue where the subscription to os changes in light/dark was removed. rust-dark-light/dark-light#68

We can still have a third mode here rather than the auto detect button. I would add "auto" - and then we can do two things.

1. When the user clicks auto, we determine the theme and apply
2. When the application starts, we do one more check to make sure the theme hasn't changed?

[NandeMD]

We can still have a third mode here rather than the auto detect button. I would add "auto" - and then we can do two things.

1. When the user clicks auto, we determine the theme and apply

2. When the application starts, we do one more check to make sure the theme hasn't changed?

Thanks for the advice! I will change the implementation to this when I'm free in a few days.

[NandeMD]

Hey @jacksongoode, sorry for the late update.

• Added a new System variant to the Theme enum and set it as the default theme.
• Updated the RadioGroup in the preferences UI to include the "System" theme option.
• If the theme is System, auto-detect system color theme on application startup.