add dark mode with toggle and system preference

[bzlibby]

I wanted to be able to apply dark mode. this automatically applies dark mode based on the user's system preference, and adds a toggle to switch between light and dark on the settings tab.

this is still in progress, but I'd appreciate any early feedback. what I plan to do still:

• fix CssBaseline import in OnnxWeb.tsx to match other imports
• move logic for checking the user's system preference and the current theme out of OnnxWeb.tsx and into utils.ts
• add a label to light/dark mode toggle
• improve logic for light/dark mode toggle based on system preference

what I'd like to do at some point, but may do as a separate PR:

• add custom theming/colors

[ssube]

Nice, this looks pretty good.

I see two minor issues, one of which you've already noted:

• mode: state.theme as PaletteMode doesn't check to make sure that state.theme is really valid, and mui will throw an error and not load the app if that is not a valid theme name
• if the user has not picked a theme yet, the toggle will not reflect their system settings and will always start with the light theme: if (state.theme === 'light') ...

For the import, I think you can import directly from the mui package, import { CssBaseline } from '@mui/material';.

[ssube]

Hi, you will probably need to rebase this in the aftermath of #208 (i18n)

[sonarcloud]

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