Accessibility & appearance settings reorganization #66183

Similar to #57008, #63454.

This is a relatively simple one:

Icon & Naming

The icon of the panel should be changed to the contrast one (pictured below), which can refer to both high contrast and dark mode, the two most common settings in this panel.

Its name should then change from “Apperance and accessibility” to “Accessibility & appearance” to both re-emphasize a11y after the icon change, and to make it a bit more glanceable and consistent with other panels.

PS: Make sure to change it in the user menu too if it's not using the same data!

High Contrast/Dark Theme

Both of these setting should work in the following model:

1. By default, they follow system preferences.
2. When toggled, they either follow the system setting if the system setting matches the toggled state, or they set an override if it does not.

This is simply the more modern/reasonable pattern. More details/reasoning in this blog post sent to me by @ShGKme :) (Although I do disagree with the post about certain things.)

Theme Color/Background

For the theme color, the first color is always the server-side default. When a preset color is matched, it shows a checkmark, otherwise, it shows a checkmark and the custom color on the last button, which opens a color picker when clicked.

They would look like this when a custom color is set:

Toast Timeout Settings

This should not be configurable. Instead, warning and error toasts should be persistent, and success ones should just disappear like they do by default. This way, no crucial information is on a timer, and users are not required to dig deep in settings.

Navigation Bar Settings

This should be replaced by inline editing, see #66141

cc @jancborchardt @marcoambrosini