mt-card: fix avatar slot positioning with title / title + subtitle

[fabianhueske]

Package: @meteor/component-library

The mt-card avatar slot is hard to use — elements placed in it are not positioned/aligned correctly, especially relative to the title and subtitle.

Proposed improvement

• Correct positioning/alignment of the avatar slot with title only
• Correct positioning/alignment with title + subtitle
• Consistent spacing between avatar and title/subtitle (token-based)
• Sensible vertical alignment regardless of avatar size
• Document both usages with examples in Storybook