feat(brand): apply the brand guidelines to meith.dev and the meith theme

[jouwdan]

Summary

Applies the September 2026 brand guidelines to the marketing site and the meith board theme, and records the guidelines in the repository.

Site (apps/web)

• Hero and closing buttons draw from tokens instead of hard-coded emerald; buttons weigh 500 and move at 160–180ms.
• Header: uppercase mono nav with the current page marked by an accent rule, a search bar with ⌘K, a light/dark/system scheme group, and the accent-filled Get started. Nav sits left, actions right, in the 80rem wide shell.
• One site-wide arrow convention: subtle at rest, accent and a 2px lift on hover; filled buttons keep the arrow in the label colour.
• Demo (demo.meith.dev) and community (forum.meith.dev) links in the header, hero, start band and footer. Copy drops ampersands and title case.

Theme (themes/meith)

• Fills all 35 slots, including sign-in, search, discovery, composer, panels, error and redirect screens, so no screen falls back to the default look.
• Semantic marks (pinned, locked, moderation, groups) sit on the greyscale ramp; the accent wash is neutral; link underlines carry 60% accent.
• Nav matches the site's mono labels, arrows stay legible on hover, post bodies run to 60rem, subforums share the forum-row grid.

Docs

• New docs/contributing/brand.md with palette, type, interface, imagery, voice and terminology, registered in the manifest and index.

Test plan

• pnpm lint, pnpm comments:check, pnpm typecheck, pnpm typecheck:site
• pnpm i18n:check, pnpm slots:check, pnpm slots:probe, pnpm depcruise
• Docs index, links, site and theme reference checks
• Theme tests and the 764-render cross-theme contract test with SHOWCASE_THEMES=1
• Site unit tests
• Checked header, hero, footer, hover states, the themed sign-in page and a forum with subforums in the browser
• Playwright e2e (marketing.spec.ts assertions updated for the scheme group, demo link and segment names, not executed locally)

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