Read the brand from openwashdata/brand 1.0.0 instead of hardcoded SCSS values

[larnsce]

The site carries the brand as hardcoded values in style/theme.scss, style/theme-dark.scss and styles.css (Atkinson Hyperlegible via a Google Fonts import, link colour #272bd1, and so on). Since 2026-09-03 the brand lives in openwashdata/brand as _brand.yml (light) and _brand-dark.yml (dark), released as v1.0.0 with a guide at https://openwashdata.github.io/brand/. Several values changed there: the typeface is now Atkinson Hyperlegible Next, owd-orange is #b04208, and there are new roles (secondary, tertiary, danger, light, dark) and a defined dark brand. The README of the brand repo names this site as the reference implementation that is due to read the files.

Tasks:

• quarto use brand openwashdata/brand (Quarto 1.9 or later) to mirror the brand into _brand/, or copy _brand.yml, _brand-dark.yml and the referenced logos
• _quarto.yml: brand: {light: _brand/_brand.yml, dark: _brand/_brand-dark.yml} and theme: {light: [brand, style/theme.scss], dark: [brand, style/theme-dark.scss]}, so the SCSS keeps the last word for layout rules
• Remove colour, font and logo values from the SCSS that the brand now provides (the Google Fonts import, the font variables, link colour, navbar colours); keep layout and component rules
• Navbar logo: the white stacked mark (_brand/logos/OWD-logo-16.png) on the purple navbar, per the guide
• Check the pages that use highlight colours as text (mint, peach, sand, rose fail contrast as text; see the guide) and switch them to the accents
• Favicon from the badge or the stacked mark

Acceptance criteria:

• No hex value that exists in _brand.yml remains in the SCSS
• Light and dark toggle render the two brands; headings, links and code match the guide

Refs openwashdata/brand#4 (the 1.0.0 round) and the reference implementation note in the brand README.

[larnsce]

Done in #141, merged to main. The site now reads _brand/_brand.yml and _brand/_brand-dark.yml from openwashdata/brand 1.0.0 with the light and dark toggle. No hex value is left in style/theme.scss, style/theme-dark.scss or styles.css; the rules use the $brand-<name> variables. Both sidebars set logo: false because the brand's medium PNG (356 by 40 pixels) looked pixelated there; larger logo exports would be a brand follow-up. The brand README's reference implementation paragraph now points here (openwashdata/brand#15).