Add "Use System Theme" setting

[uday-rana]

I'd like a setting that makes the app theme change dynamically when the system theme changes.

I think setting useSystemColorMode to true if the setting is toggled on would work:

chatcraft.org/src/theme.ts

Lines 5 to 10 in c2f7165

	config: {
	// Initialize dark/light mode with system pref, but remember choice
	// via localStorage setting
	initialColorMode: "system",
	useSystemColorMode: false,
	},

Maybe also hide/disable the theme toggle button when this is on?