[Popover]: add Header popover variant under regular Popover

[karolin-kivisikk]

Design: https://www.figma.com/design/jWiRIXhHRxwVdMSimKX2FF/TEDI-READY-2.45.70?node-id=5797-117364&t=JB46lWnLynhVwfHW-4

1. Update Popover stories to include Header popover examples (With prominent border) and unify with React Popover stories.
2. Fix popover offset when withBorder variant is used. Currently withBorder Popovers are placed on the Header.

DoD

• The component is developed using best practices, conventions, and modern frontend trends
• The component follows BEM naming conventions (if applicable)
• The component implementation matches the Figma design, including all defined states and interactions
• The component supports responsiveness and scales across breakpoints (if applicable)
• The component does not introduce runtime errors or warnings
• Linter passes
• Component has export if needed
• The component has unit tests with at least 80% code coverage
• CodeCov passes minimum coverage threshold
• The component has passed code review
• The component has passed Chromatic visual review
• Storybook stories are created:
  • Cover all Figma variations
  • Include edge cases, empty/error states
• Component’s API (props, slots, events) is documented
• ZeroHeight is updated with design/development guidelines
• External dependencies used in the component are documented in Confluence with rationale.
• Semantic commit format is used (for semantic-release)
• External libraries comply with validated open source licenses
• Component is versioned correctly and ready for release if required
• Update statuses page in Zeroheight