chore(deps): bump @astrojs/preact 6, @primer/react-brand 0.69, @types/node 26

Stacks three passing Dependabot bumps (#110, #109, #111) into one branch
with a single consolidated lockfile. Verified locally with Node 22:
astro check, astro build, and Playwright tests all pass.

Excludes #112 (astro 7): it pulls Vite 8 (rolldown), whose built-in
postcss-import cannot resolve Tailwind v4's @import "tailwindcss",
which needs a dedicated migration.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: jldeen