Update dependencies to address security vulnerabilities (#140)

Updates:
- gatsby: 5.13.7 -> 5.15.0
- react/react-dom: 18.2.0 -> 18.3.1
- postcss: 8.4.41 -> 8.5.6
- tailwindcss: 3.4.9 -> 3.4.18
- @tailwindcss/typography: 0.5.15 -> 0.5.19
- autoprefixer: 10.4.20 -> 10.4.22
- gatsby-plugin-google-gtag: 5.13.1 -> 5.15.0
- gatsby-plugin-postcss: 6.13.1 -> 6.15.0
- @types/node: 22.9.0 -> 22.15.0
- @types/react: 18.2.67 -> 18.3.27
- @types/react-dom: 18.2.22 -> 18.3.7
- typescript: 5.4.2 -> 5.8.3

Reduces npm audit vulnerabilities from 28 to 15 (remaining are
Gatsby internal dev tooling dependencies that require breaking changes).

Co-authored-by: Claude <noreply@anthropic.com>

Author: georgeguimaraes