chore(deps): update dependencies to latest

Routine refresh on top of v4.0.0:

- @astrojs/mdx 5.0.6 -> 6.0.0 (major; builds clean against astro 6.4)
- astro 6.3.3 -> 6.4.0
- concurrently 9.2.1 -> 10.0.0 (major; dev-only parallel script runner)
- @typescript-eslint/eslint-plugin + parser + typescript-eslint
  8.59.3 -> 8.60.0
- stylelint 17.11.1 -> 17.12.0
- sass 1.99.0 -> 1.100.0
- terser 5.47.1 -> 5.48.0
- postcss 8.5.14 -> 8.5.15

Full production build (lint + compile + bundlewatch) passes and
npm audit reports 0 vulnerabilities.