Introduce a new, modern user profile interface with a dedicated
statistics grid and improved visual hierarchy.
- Implement `.ifnex-user-profile` and `.ifnex-profile-header` components
- Add responsive statistics grid with support for warning, info, and
success color variants
- Add dark mode support via `prefers-color-scheme` media queries
- Refactor profile shortcode to use new CSS classes instead of inline
styles
- Remove legacy inline-styled multi-currency credit table in favor of
the new design system
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