Top Toolbar: Refine responsiveness and apply across mobile

[jasmussen]

The top toolbar mode option puts the block toolbar in the top center of the main toolbar.

As the screen grows smaller, we can refine this responsiveness as shown here:

• At 960px, the document title contracts, and the rightmost tools are hidden as well, when a block is selected.
• At 480, the document title contracts further, and hides the shortcut hint.
• On mobile and embedded, the document title is entirely hidden.

Related: #51711.

[jasmussen]

Closing this one for now, as separate inititatives have caused drift with this one and are being tracked elsewhere.