site: landing scroll jump in theme section, livelier hero app, sketchbook polish #122

Landing page (site/src/pages/index.astro, demos, site/src/theme.ts):

1. Scroll jumps/sticks in the theme section ('One hex in, a whole theme out') while switching swatches/style.
2. Hero example app (Northwind) should feel alive (live ticks, chart animates in, status, toggle, toast); respect reduced motion.
3. Orange glow in hero card corner clashes with the sketchbook theme.
4. Polish sketchbook theme: calmer contrast/grid, plainer copy, consistent buttons, less code above the fold, dark+light.