Add dark mode with the Obol-branded palette

[ksale001]

Summary

• Adds a light/dark toggle (sun/moon in the top bar) using the Obol-branded dark palette from the original standalone prototype. Defaults to light, remembers your choice, and follows you onto the methodology page.
• Dark values live in a single :root[data-theme="dark"] block in theme-tokens.css that overrides the --theme-* source tokens, so the existing alias layer recomputes on its own. An inline script in _document sets the theme before first paint so dark users never flash white.
• The pizza graphic now reads CSS variables instead of hardcoded hex so it recolors with the theme.
• Also folds in two small fixes found while testing: the stage-ladder pill labels (Getting started / Safety / Liveness) now center vertically, and the first question has a Back button that returns to the intro without losing your answers.

Deferred (not in this PR)

• The 729 static OG share images stay light for now. They're pre-rendered at build time, so a viewer's theme can't reach them. We can generate dark variants later if we want them.
• Light mode keeps its current colors. A separate pass to rework the light palette is on my list.

How to test it

• yarn test and yarn lint pass (ran locally)
• Toggle dark on, reload mid-assessment, confirm no white flash and the choice sticks
• Check the pizza (plate, slices, center STAGE label) and a few questions in dark
• Methodology page toggles too