fix(Modal): aside height calculated incorrectly

Description

This PR introduces a fix on the Modal component regarding the height of the aside element.

The Storybook setup did not contain any CSS reset file; this means that the box-sizing property was implicitly set to content-box for certain elements. The aside element is an example of this: setting its height to 100% would result in it taking its container's height plus its own vertical padding.

Modal

updated aside element to have box-sizing: border-box and height set to 100%;
added CSS reset to storybook preview
updated Modal stories to improve the ease of use

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mia-platform / design-system