Tasks — add-reference-web-dark-mode

1. Brand tokens

• 1.1 Add --surface-tint light/dark token in packages/pdpp-brand/base.css and replace literal white keyword in color-mix(...) expressions and the hardcoded oklch(1 0 0) in the body gradient.
• 1.2 Add html.dark { ... } override block in packages/pdpp-brand/base.css with full dark token palette (background, foreground, card, popover, primary, secondary, muted, accent, destructive, border, input, ring, success, warning, edu-fg, success-wash, success-wash-strong, primary-wash, verified-wash, warning-wash, human, human-wash, surface-tint).
• 1.3 Update packages/pdpp-brand/docs.css to use --surface-tint rather than literal white in the color-mix mixers it uses for sidebar tint, prose body color, prose code background, and table header.
• 1.4 Override scrollbar thumb color under html.dark so the oklch(0 0 0 / …) light-mode thumb does not vanish on dark.

2. Theme runtime

• 2.1 Add inline pre-hydration theme script in apps/web/src/app/layout.tsx (head, dangerouslySetInnerHTML) that reads localStorage + prefers-color-scheme and applies html.dark, data-theme, and style.colorScheme before paint.
• 2.2 Add apps/web/src/components/theme/theme-provider.tsx (client) that exposes { theme, resolvedTheme, setTheme } via context, persists to localStorage, and listens for system preference changes when in "system" mode.
• 2.3 Add apps/web/src/components/theme/theme-toggle.tsx (client) — a small icon button cycling light → dark → system, with accessible labels.

3. Mount points

• 3.1 Wrap RootProvider's children with ThemeProvider in apps/web/src/app/layout.tsx.
• 3.2 Place <ThemeToggle /> in the dashboard Topbar (apps/web/src/app/dashboard/components/shell.tsx).
• 3.3 Place <ThemeToggle /> in the public SiteHeader (apps/web/src/components/site-header.tsx).

4. Surface polish

• 4.1 Audit dashboard surfaces (records, runs, deployment, search, overview) for any hardcoded color utilities that would regress in dark mode. Convert to semantic tokens or add dark: variants where unavoidable.
• 4.2 Verify shadcn primitives (button, badge, card, dialog, popover, input, select, tooltip) render correctly in dark mode under the new tokens — they already use dark: variants in places, so most just needs the underlying tokens to be right.

5. Validation

• 5.1 pnpm --dir apps/web run types:check
• 5.2 pnpm --dir apps/web run check
• 5.3 pnpm --dir apps/web run build
• 5.4 openspec validate add-reference-web-dark-mode --strict
• 5.5 openspec validate --all --strict

6. Deferred

• 6.1 Marketing / hero polish in dark mode (illustrations/washes use light-mode-baked gradients; legible but not a deliberate dark composition).
• 6.2 Dedicated docs toggle inside Fumadocs chrome — currently controlled by the dashboard/site header toggle via the shared html.dark class.
• 6.3 /palette and /design reference pages beyond falling out of the new tokens.

Acceptance checks

• Booting the dashboard with the OS in dark mode and no stored preference renders dark on first paint with no flash.
• Toggling the theme changes both the dashboard chrome and the embedded /docs and /reference surfaces in the same window.
• Reload preserves the explicit choice; clearing storage falls back to system preference.
• Status badges (online/offline, success/destructive) remain identifiable at a glance in dark mode.