Implement Tree Editor with visual preview and documentation updates

Tree Editor Features:
- Zustand store with immer middleware and zundo for undo/redo
- Form-based node editing (Decision, Action, Solution types)
- Visual tree preview with solution connection indicators
- NodePicker with type-grouped dropdown (Decisions/Actions/Solutions)
- SharedLinksMap for detecting nodes with multiple sources
- Modal component with scrollable body, fixed header/footer

New Components:
- TreeEditorLayout, TreeMetadataForm, NodeList, NodeEditorModal
- NodeFormDecision, NodeFormAction, NodeFormResolution
- DynamicArrayField, NodePicker
- TreePreviewPanel, TreePreviewNode

Documentation:
- Updated README.md status to Phase 2
- Added Tree Editor details to CURRENT-STATE.md
- Added modal/Zustand lessons to LESSONS-LEARNED.md
- Updated file structure in CLAUDE-SETUP.md
- Added Tree Editor progress to PROGRESS.md

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/pages/index.ts

@@ -2,5 +2,6 @@ export { default as LoginPage } from './LoginPage'
 export { default as RegisterPage } from './RegisterPage'
 export { default as TreeLibraryPage } from './TreeLibraryPage'
 export { default as TreeNavigationPage } from './TreeNavigationPage'
+export { default as TreeEditorPage } from './TreeEditorPage'
 export { default as SessionHistoryPage } from './SessionHistoryPage'
 export { default as SessionDetailPage } from './SessionDetailPage'