Fix primary-button contrast and reuse it for the publication CTA
The publication CTA hand-rolled its own `.author__cta` button instead of
the design system's `.btn .btn--primary`. Point it at the shared button
so its styling and hover stay in sync with the rest of the app; this also
lets the author page drop its bespoke `is:global` block (the .btn classes
are already global via Base.astro).
White text on `--sun` only reaches ~2.5:1 contrast (1.9:1 in dark mode,
where the accent is light) — below WCAG AA. Introduce `--btn-primary` /
`--btn-primary-hover` fills, a deeper sun tuned to clear 4.5:1 with white
text in both color schemes, and use them for `.btn--primary` and the
dashboard's `.dash__new`.