feat: restyle themes to be visually distinct

- Minimal: Swiss Brutalist — fullbleed hero, true B&W, zero-gap grid, Geist font
- Editorial: Cinematic Magazine — warm charcoal, Bodoni Moda serif, slow transitions
- Classic: Gilded Gallery — gold accents, Cinzel headings, passepartout, ornamental dividers

Updated config presets, per-theme color palettes, ~550 lines of scoped CSS overrides,
data-preset attribute on html element, responsive breakpoints, screenshots, and docs.

app/layout.tsx

@@ -57,6 +57,7 @@ export default function RootLayout({ children }: { children: React.ReactNode })
       lang="en"
       suppressHydrationWarning
       style={themeVars as React.CSSProperties}
+      data-preset={theme.preset}
       data-grain={theme.grain}
       data-header-dot={theme.headerDot}
       data-photo-frame={theme.photoFrame}