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-lock.json

@@ -11,11 +11,13 @@
         "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": {
@@ -3039,6 +3041,16 @@
         "node": ">= 4"
       }
     },
+    "node_modules/immer": {
+      "version": "11.1.3",
+      "resolved": "https://registry.npmjs.org/immer/-/immer-11.1.3.tgz",
+      "integrity": "sha512-6jQTc5z0KJFtr1UgFpIL3N9XSC3saRaI9PwWtzM2pSqkNGtiNkYY2OSwkOGDK2XcTRcLb1pi/aNkKZz0nxVH4Q==",
+      "license": "MIT",
+      "funding": {
+        "type": "opencollective",
+        "url": "https://opencollective.com/immer"
+      }
+    },
     "node_modules/import-fresh": {
       "version": "3.3.1",
       "resolved": "https://registry.npmjs.org/import-fresh/-/import-fresh-3.3.1.tgz",
@@ -4790,6 +4802,24 @@
         "zod": "^3.25.0 || ^4.0.0"
       }
     },
+    "node_modules/zundo": {
+      "version": "2.3.0",
+      "resolved": "https://registry.npmjs.org/zundo/-/zundo-2.3.0.tgz",
+      "integrity": "sha512-4GXYxXA17SIKYhVbWHdSEU04P697IMyVGXrC2TnzoyohEAWytFNOKqOp5gTGvaW93F/PM5Y0evbGtOPF0PWQwQ==",
+      "license": "MIT",
+      "funding": {
+        "type": "individual",
+        "url": "https://github.com/sponsors/charkour"
+      },
+      "peerDependencies": {
+        "zustand": "^4.3.0 || ^5.0.0"
+      },
+      "peerDependenciesMeta": {
+        "zustand": {
+          "optional": false
+        }
+      }
+    },
     "node_modules/zustand": {
       "version": "5.0.10",
       "resolved": "https://registry.npmjs.org/zustand/-/zustand-5.0.10.tgz",