feat(styleguide): add sun & moon halo lab sub-page

[johannesjo]

What

Adds a halo lab sub-page to the styleguide for comparing sun/moon halo & glow treatments side by side and over the live page gradient.

• Grid view — 12 sun + 12 moon halo studies, each rendered on the live --background-gradient so a pale halo is judged in context, not on a flat card.
• Drag stage — click any study to open it full-screen and drag the disc across every band of the gradient; step through variants with ‹ / › and toggle dark mode to test the night sky.
• Reached from a new "Open halo lab →" entry under the styleguide's Interactions section; "← styleguide" returns.

How

• StyleguideHalos.tsx — the sub-page (grid + full-screen drag stage). One HaloDisc component backs both views; only the --halo-* custom properties change per variant.
• haloVariants.ts — the 24 recipes, expressed purely as CSS-variable strings (--halo-disc / --halo-shadow / --halo-wash) so a winner ports into Sun.scss by copying three strings. Moons set --halo-wash: none to keep the night scene free of warm wash.
• styleguideHalos.module.scss — grid, disc, and stage styling; halo breathes via opacity only (compositor-cheap) and respects prefers-reduced-motion.
• Styleguide.tsx — a showHalos signal swaps the page for the lab via <Show>; reuses <Btn> and the shared BackgroundTransition.

Notes

• Styleguide-only — no production interaction code is touched.
• Rebased onto current main (resolved against the new let-go demo and dashboard-simulation sub-page); all three styleguide entry points coexist.
• Typecheck clean for the new files.

[github-actions]

🌅 minded preview

This PR is deployed and ready to explore:

🎨 Styleguide — components & interactions	https://johannesjo.github.io/minded/pr-preview/pr-56/
🌥️ Dashboard simulation — the full app shell	https://johannesjo.github.io/minded/pr-preview/pr-56/dashboard.html

_{Updated for c1ec166. Auto-removed when the PR closes.}