extend app.css with the every-layout style primitives (container,
stack, cluster, center) used to compose page shells, plus a small
component layer: site-header, btn (primary/secondary/danger), card
and card-link, list-cards, modal, error and a few utility classes.
no template changes here - templates start adopting these classes
in the per-page styling commits that follow.
introduce a single shared stylesheet under public/css. this first
section covers a minimal reset, warm/readable design tokens (color,
typography, spacing, radii, shadows), and base element styling for
headings, links, lists, forms, and buttons. layout primitives and
component classes follow in subsequent commits.
