feat: add PageHeader component and centralize page spacing

- Create reusable PageHeader component (emoji + title + subtitle + actions)
- Use PageHeader in sessions, teams, users, objectives pages
- Centralize vertical padding in layout (py-6) and remove per-page py-* values

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/components/ui/index.ts

@@ -9,6 +9,7 @@ export { EditableMotivatorTitle } from './EditableMotivatorTitle';
 export { EditableYearReviewTitle } from './EditableYearReviewTitle';
 export { EditableWeeklyCheckInTitle } from './EditableWeeklyCheckInTitle';
 export { EditableWeatherTitle } from './EditableWeatherTitle';
+export { PageHeader } from './PageHeader';
 export { Input } from './Input';
 export { ParticipantInput } from './ParticipantInput';
 export { Modal, ModalFooter } from './Modal';