Split 1257-LOC client page into _types.ts plus nine components under
_components/ (TabNavigation/StatCard/FilterBar in shared, CourseCard,
EnrollmentCard, CertificatesTab, EnrollmentEditModal, CourseEditModal,
SettingsTab, and PageSections for header actions and empty states).
Behavior preserved exactly; page.tsx is now a thin wiring shell.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
