feat(design-system): Stage 4A — design system expansion and shared widget migration
- Migrate EmptyStatePanel, SectionHeader, SummaryCard from kell_web into design_system as KcEmptyState, KcSectionHeader, KcSummaryCard
- Add KcTypography shared typography scale with full Material 3 text style hierarchy
- Add KcBreakpoints responsive layout breakpoint utilities (compact/medium/expanded/large)
- Add KcLoadingState and KcErrorState shared state widgets
- Update kc_theme.dart to use KcTypography.applyKcTypography()
- Update kell_web dashboard_page.dart to use design_system widgets directly
- Replace kell_web shell widget files with backward-compatible typedef re-exports
- Expand design_system tests from 3 to 41 (all passing)
- All existing tests passing: design_system 41/41, feature_wordpress 294/294, kell_web 24/24
- dart analyze clean across design_system and kell_web
Mike Kell
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