feat(chartkit-core): let one chart name its own colors

[kkukielka]

Palettes live in the site's CSS, which is the right default: every chart stays
on brand through a restyle and nobody types a hex code into a CMS. It also left
the chart that needs one series picked out with no way to say so - the last item
in Chartkit's Next.

The shape

options.colors is sparse. null or a missing entry leaves that index to the
stylesheet, so highlighting the second of five series is [null, '#c00'] rather
than five colors that would then be frozen against the next restyle.

options: { colors: [null, '#d1373b'] }

An index means whatever the legend is naming - a series in a bar, line or area
chart, a slice in a pie or donut - so one Palette function covers every chart
type and the legend swatches cannot drift from the marks.

The one decision worth arguing about

A color named in the spec is written into the markup literally, not as a
var() fallback, so it beats the page's stylesheet. That is the opposite of
Chartkit's default and it is the point: a chart naming a color is overruling the
house style deliberately, and a house style that could silently repaint it would
make the option useless in the case it exists for.

Validation

Values are checked against an allowlist of CSS colors. Attributes are already
escaped, so this is not about markup injection - url(https://…) is a valid
paint value that would make a chart fetch from a third party as it renders, and
a typo is better reported than drawn as black.

Editor

A swatch per series under the data grid. Two details carry the "your stylesheet
is still in charge" decision:

• Only entries someone sets are stored - setColor pads with null, trims
  trailing holes and drops the array when the last override goes, so an
  experimented-with chart ends up identical to one that never had colors.
• A native color input has no "unset", so each set swatch gets a cross that
  hands that series back to the site. Without it the first click is
  irreversible.

Verified

110 tests across core and editor, including the sparse-array trimming and the
rejected paint values.

The gallery gains a second copy of one fixture rendered inside a page that
does define a palette. Every other card renders against no stylesheet at
all, so without that control a color that quietly stopped deferring to the site
would look identical. Checked in the browser:

• unthemed card: baseline blue (fallback), highlight #d1373b, other salmon
• themed card: baselines take the site's teal and plum, highlight keeps its red
• donut: colors key to slices, legend agrees with the ring

And in the Strapi admin, live: setting a swatch repaints only that series and
leaves the others as var(--chart-series-N, …); the cross appears only on a set
entry and clearing the last one removes colors from the spec entirely.

Versions

Three packages gained the feature and are committed under their own scopes, so
nx release gives each a minor rather than an indirect patch - core 0.4.0,
editor 0.3.0, plugin 0.3.0. The three renderers take a patch as dependents,
which is right: they pass the SVG through.