[6.x] Correct the Bard full-screen width. The usual max-w-page is based on …

[jaygeorge]

Description of the Problem

When Bard is full-screen, the max-width is set to max-w-page, the same as the publish form, which theoretically should work well.
However, the publish form has a sidebar and a grid layout, which means the actual width of a Bard field is narrower, at ~65rem, leaving an ideal, readable width.

What this PR Does

• Sets the Bard max-width to 65rem in full-screen mode, matching the reading width in non-full-screen mode.
• Closes Normal text width in Bard Fullscreen mode is fairly wide and difficult to read #14229

Bard reading standard reading width

Full-screen (before)

Full-screen (after)

Note how the line breaks at the same point

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I considered adding an Expand Layout toggle control here, but then I reasoned it's unlikely to be useful when writing prose.

How to Reproduce

1. Observe that Bard's reading width is slightly wider in full-screen mode.