fix: remove chart container border

[Mirochill]

Summary

• remove the default border from chart containers so rendered charts no longer get a thin frame
• add an Unreleased version history note for the styling change

Validation

• git diff --check HEAD~1..HEAD
• static check that the old border: 1px solid #343131 rule is no longer present
• Tests not run locally

Closes #24

This targets main; I can retarget it if a version branch is preferred.