[6.x] Fullscreen mode improvements for Bard and Markdown

[JayGeorge]

A few improvements to the way fullscreen is handled, both across Markdown fields and Bard fields…

• The blue focus outline is not shown when you're focused in the writing area, making it easier to … focus, ironically.
• The writing area now focuses automatically when switching to full screen. Previously you needed to click into the writing area
• More consistent aesthetics - toolbar spacing and border-radii