Link and Grid refinements

A larger Link size, consistent Grid spacing, and a clearer external-link API.

New features

• Added size="lg" to Link.
• Added implicit track controls and item border radius to Grid.

Bug fixes

• Fixed Grid gaps to use the same base unit as Stack, including axis-specific gaps and fallback values.

Breaking changes

• Renamed the Link external prop to isExternal.
• Removed Interactive props that had no effect on Link from its public API.

// Before
<Link external href="https://vortex.design" />

// After
<Link isExternal href="https://vortex.design" />