Outline chevron buttons were matched by a generic `.cms-outline li
button { width: 100% }` rule, so each expand/collapse arrow took a full
row and the tree looked stacked and broken. Scope row styling to
`.cms-orow__main`, fix the chevron to 22px, add rotating chevrons to the
section headers, and turn the blocks palette from centered narrow cards
into left-aligned rows.
Add a split toggle to the panel header: outline stays visible in a left
column while Blocks/Nodes switch as tabs in a right column, so a drag
source and its outline drop gaps are on screen together. Choice and
library tab persist per profile; stacked mode is unchanged.
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