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/package.json

@@ -13,11 +13,13 @@
     "axios": "^1.13.4",
     "class-variance-authority": "^0.7.1",
     "clsx": "^2.1.1",
+    "immer": "^11.1.3",
     "lucide-react": "^0.563.0",
     "react": "^19.2.0",
     "react-dom": "^19.2.0",
     "react-router-dom": "^7.13.0",
     "tailwind-merge": "^3.4.0",
+    "zundo": "^2.3.0",
     "zustand": "^5.0.10"
   },
   "devDependencies": {