chore(deps): bump postcss to 8.5.26

Updates postcss from 8.5.15 to 8.5.26 to address a high severity
security vulnerability. It was first attempted by dependabot in PR #5953
but that PR also included a vue-loader bump that would require a Vue 3
migration, which is out of scope for now.

Also bumps nanoid from 3.3.12 to 3.3.18, as postcss 8.5.26 raises its
required range to ^3.3.17.

Assisted-by: ClaudeCode:claude-opus-5
Signed-off-by: Elizabeth Danzberger <elizabeth@elzody.dev>

Author: elzody