Add Reference Web Dark Mode

The reference web app is light-only. The owner uses the dashboard as a sustained operator surface (records, runs, deployment diagnostics, search), and prolonged light-mode use is uncomfortable. The brand and shadcn primitives already encode every color through semantic CSS variables (--background, --foreground, --muted, --border, --primary, --success, --destructive, etc.) and Tailwind 4 already exposes a dark variant via @custom-variant dark (&:is(.dark *)). The missing piece is an actual .dark token set, a flicker-free toggle, and a few legacy color-mix(... white) mixes in the brand CSS that bake the page background being white.

A dashboard dark theme that an operator can stare at for hours. - Preserve PDPP's existing visual language: cool blue primary, warm "human" accent, success/warning/destructive semantics. Do not produce the generic saturated-purple-on-pure-black look. - Avoid hydration flash in App Router on first paint. - Make the toggle reachable from anywhere in the live operator surface.

The PDPP reference web app SHALL support an explicit dark theme alongside its existing light theme, the dashboard SHALL be usable for sustained operator sessions in either theme, and the theme choice SHALL apply to dashboard, docs, and reference public surfaces inside the same browser session.