Enlarge topbar brand mark to 48px

Restyle tabs as a contained segment switcher ...

The tab list becomes a framed panel with inner padding, and
tabs are rounded chips inside it: subtle hover highlight,
filled primary active state, hairline separators removed.
Matches the demo theme switch style.

Soften Verdant palette ...

Desaturate the neutrals, text tones and accents so the green
reads calmer: barely-tinted darks and text, softer mint
secondary, muted lime accent, desaturated teal info.

Add Verdant color theme with demo switcher ...

Palette variables become !default so themes can configure them.
kit-verdant.scss / demo-verdant.scss compile the green-shifted
palette (green-tinted neutrals, mint secondary, lime accent,
teal info) into kit-verdant.css / demo-verdant.css. Both demos
get a fixed theme switch in the corner that swaps the theme
stylesheets and persists the choice. The default Tokyo Night
theme is unchanged.