feat(theme): let the merchant choose the form color scheme

PAGE_CSS carried an unconditional `@media (prefers-color-scheme: dark)`
block that redefined --bg, --surface, --text and --border. The merchant
theme override only sets accent, button and typography tokens, so a
visitor whose OS is in dark mode saw a dark card with the merchant's
button colour on it, and no setting could change that. The form is
rendered in a modal on top of the shop theme, which the app cannot read
and which is almost always light.

Move the dark palette out of PAGE_CSS into DARK_VARS and let renderShell
apply it according to a new "themeScheme" setting: light (default), dark,
or auto (follows prefers-color-scheme, the previous behaviour).

The cascade is now base light palette -> chosen scheme -> merchant
override, so custom accent and button colours win under either scheme.
`color-scheme` and its meta tag follow the same setting, which keeps the
native controls consistent.

Existing rows have themeScheme NULL and fall back to the light default.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Mv83a29B4eFv5ixoj6PoE1

app/prisma/migrations/20260710163000_theme_scheme/migration.sql

@@ -0,0 +1,3 @@
+-- Schema colore del form: "light" | "dark" | "auto".
+-- NULL sulle righe esistenti: l'app applica il default (light).
+ALTER TABLE "Settings" ADD COLUMN "themeScheme" TEXT;