feat(components): add keepMounted prop to DialogContent

[Saikat-D023]

Summary

Adds a keepMounted prop to DialogContent, passing it through to DialogPortal.

Problem

In Base UI dialogs, the content is unmounted when closed, which causes issues in scenarios where the DOM needs to persist (e.g. state reset, animations, or integrations relying on mounted elements).

Solution

Expose a keepMounted prop on DialogContent so consumers can control this behavior.

Notes

• Applied consistently across base-* styles
• Added example usage in docs

Related to #10483

[vercel]

@Saikat-D023 is attempting to deploy a commit to the shadcn-pro Team on Vercel.

A member of the Team first needs to authorize it.