Secondary color for site branding

Site branding gains a new Secondary color field, alongside the existing brand color.

It's used for the shopper's own message bubble and the numbered badge on a clarifying question (not the header or buttons, which keep using your brand color). Pick a hex color, or leave it unset to keep the default neutral gray.

Like every other branding field, it's optional and applies wherever branding already showed up: the playground's chat panel, and the public agent page.