feat(ui): separate toolbars from the content below with dividers

[JunkFood02]

What:

• Add two dividers between the toolbars and the content.
• The top divider only appears when the article is being scrolled up.

Why:

• To ensure there's suffient contrast between content and toolbars. When the tonal elevation of toolbars is set to 0 (default value), the toolbars actually uses the same container color as the content beneath.
• Consider removing this preference and fixing the tonal elevation to 0

Screenshots