Allow ColorInputView swatches to be {value, text} objects in addition
to plain color strings. The text is shown as the swatch tooltip; the
value is the color that gets applied. Swatches are now deduplicated by
value so a labelled preset and a plain color with the same value do not
appear twice. Plain string swatches keep their existing behavior.

Merge theme-provided background color presets ahead of colors already
used in other sections, and offer the combined list as swatches for the
backdrop, card surface and split overlay color inputs. Presets surface
with their inline name as the swatch tooltip; picking one stores the
plain color value.

Allow ColorInputView swatches to carry an optional group key. Consecutive
swatches with a different group are separated by a divider inside the
picker drop down, letting callers visually distinguish sets of colors
(e.g. theme presets vs. colors already in use) without labels. Plain and
ungrouped swatches keep rendering without a divider.

Tag theme background color presets and colors already used in other
sections with separate group keys before passing them to the backdrop,
card surface and split overlay color inputs. The picker then separates
the two sets with a divider instead of showing one flat list.

Add a section to the Custom Colors and Dimensions guide describing the
presets.background_colors theme option: how it differs from palette
colors (copied by value, not referenced), why names are stored inline,
and how presets surface in the section background color picker.