M19 — Page skins

[julia-shtal]

Context. book.css currently has one fixed palette + page-stack texture,
entirely driven by CSS custom properties (README: "no hard-coded colours
elsewhere").

Scope

1. Introduce a data-theme attribute on the app root with 2–3 named skins
   (e.g. parchment [current default], night, linen), each just
   redefining the existing CSS custom properties — no new component logic.
   If implementing this surfaces a component that hardcodes a color instead of
   using the token system, fix that component as part of this task (it's a
   pre-existing violation of the stated CSS convention, not scope creep).
2. Settings view: a theme picker next to font family/size, applied live
   without restart — same pattern as the other "live settings."
3. Persisted in settingsStore/userData/settings.json.

Out of scope v1: a free-form/custom color picker — fixed curated skins only.

Acceptance criteria

• Switching themes in Settings updates the editor immediately, no restart.
• Every themed surface uses CSS custom properties — no leftover hardcoded colors.
• Selected theme persists across restart.