feat: add form and layout components to shared-ui (Tier 6b)

New molecules:
- ModalFooter: Standardized modal footer with button layout
- DataCard: Generic card for displaying data items (memos, decks, etc.)
- PageHeader: Standardized page header with title, description, actions

New organisms:
- FormModal: Modal with built-in form handling, validation, loading states

All components use Svelte 5 snippets for flexible slot patterns.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

packages/shared-ui/src/index.ts

@@ -20,8 +20,11 @@ export { SkeletonBox, SkeletonText } from './molecules';
 // Feedback
 export { EmptyState } from './molecules';
 
+// Layout
+export { ModalFooter, DataCard, PageHeader } from './molecules';
+
 // Organisms
-export { Modal, ConfirmationModal, AppSlider } from './organisms';
+export { Modal, ConfirmationModal, FormModal, AppSlider } from './organisms';
 export type { AppItem } from './organisms';
 
 // Navigation