Create generic popover component

The resilience designs call for the following 'popover':

To implement this, a good first step would be to create a generic popover component similar to the [Base UI Popover](https://base-ui.com/react/components/popover), but with basic Compound styles applied.

The resulting component can live in this repository rather than upstreaming to Compound Web, because for now this seems to be a one-off thing for Element Call.

Some requirements:
- Supports an arbitrary `trigger` button
- Requires a `title` string
- Accepts arbitrary content (`children`)
- Applies font/body/sm/regular and color/text/secondary tokens to its textual content

Nice to have, not strictly required:
- Is built with [Base UI](https://base-ui.com/) rather than Radix and Vaul (as we would like to have a proof point for replacing Radix with Base UI more widely in Compound)
- Displays as a bottom sheet on mobile devices in portrait orientation (reference the following [mobile](https://www.figma.com/design/aOEkaJtaBmPy058V7uoqVr/Element-Call-Updates---2026?node-id=1391-54390) [designs](https://www.figma.com/design/aOEkaJtaBmPy058V7uoqVr/Element-Call-Updates---2026?node-id=1426-13004))

Start by reading the linked Base UI Popover reference and the issue’s resilience and mobile designs, then inspect this repository’s existing Compound-styled components to find the appropriate entry point. Done means a generic component supports an arbitrary trigger, required title, arbitrary children, and the specified text tokens; mobile bottom-sheet behavior is optional.

Written by the indexing model from the issue text.