Add built-in padding to the Core Catalog items

[jacobsimionato]

A2UI intentionally doesn't not let the LLM choose padding. Instead, we the renderer is responsible for this.

Our core catalog items have no built-in padding, so everything looks bad in the simple chat app.

How about these rules:

• Leaf components (e.g. text, image etc) add padding around themselves on every dimension by default. They can be special cased via usage hints to not do this, e.g. a "header" image has no top or side padding, but does have bottom padding.
• Container components which render nothing add no padding (e.g. Row, Column etc)
• Container components which have a border (e.g. Card) add padding around themselves, but not around their children

[jacobsimionato]

See example of poor padding: