set screen width and center content

[jduss4]

We need to cap the width of the display at 1200px / 75 rem and center everything in the window.

This came up during a conversation with @beepdotgov and I wanted to make an issue before it became lost to time.

Acceptance Criteria

• [ ] Widescreen is capped at specified width, content is centered

[beepdotgov]

Thanks for logging this, @jduss4. (Updated Figma with a comment/note as well.)