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/router.tsx

@@ -6,6 +6,7 @@ import {
   RegisterPage,
   TreeLibraryPage,
   TreeNavigationPage,
+  TreeEditorPage,
   SessionHistoryPage,
   SessionDetailPage,
 } from '@/pages'
@@ -38,6 +39,14 @@ export const router = createBrowserRouter([
         path: 'trees',
         element: <TreeLibraryPage />,
       },
+      {
+        path: 'trees/new',
+        element: <TreeEditorPage />,
+      },
+      {
+        path: 'trees/:id/edit',
+        element: <TreeEditorPage />,
+      },
       {
         path: 'trees/:id/navigate',
         element: <TreeNavigationPage />,