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/components/tree-editor/NodeEditorModal.tsx

@@ -0,0 +1,85 @@
+import { Modal } from '@/components/common/Modal'
+import { useTreeEditorStore } from '@/store/treeEditorStore'
+import { NodeFormDecision } from './NodeFormDecision'
+import { NodeFormAction } from './NodeFormAction'
+import { NodeFormResolution } from './NodeFormResolution'
+import type { TreeStructure } from '@/types'
+
+interface NodeEditorModalProps {
+  node: TreeStructure
+  onClose: () => void
+}
+
+export function NodeEditorModal({ node, onClose }: NodeEditorModalProps) {
+  const { updateNode, validationErrors } = useTreeEditorStore()
+  const nodeErrors = validationErrors.filter(e => e.nodeId === node.id)
+
+  const handleUpdate = (updates: Partial<TreeStructure>) => {
+    updateNode(node.id, updates)
+  }
+
+  const getTitle = () => {
+    switch (node.type) {
+      case 'decision':
+        return 'Edit Decision Node'
+      case 'action':
+        return 'Edit Action Node'
+      case 'solution':
+        return 'Edit Solution Node'
+      default:
+        return 'Edit Node'
+    }
+  }
+
+  const footerContent = (
+    <div className="flex justify-end">
+      <button
+        type="button"
+        onClick={onClose}
+        className="rounded-md bg-primary px-4 py-2 text-sm font-medium text-primary-foreground hover:bg-primary/90"
+      >
+        Done
+      </button>
+    </div>
+  )
+
+  return (
+    <Modal isOpen={true} onClose={onClose} title={getTitle()} size="lg" footer={footerContent}>
+      {/* Node ID display */}
+      <div className="mb-4 text-xs text-muted-foreground">
+        Node ID: <code className="rounded bg-muted px-1 py-0.5">{node.id}</code>
+      </div>
+
+      {/* Validation errors */}
+      {nodeErrors.length > 0 && (
+        <div className="mb-4 space-y-1">
+          {nodeErrors.map((error, i) => (
+            <div
+              key={i}
+              className={`rounded-md px-3 py-2 text-sm ${
+                error.severity === 'error'
+                  ? 'bg-destructive/10 text-destructive'
+                  : 'bg-yellow-500/10 text-yellow-600 dark:text-yellow-400'
+              }`}
+            >
+              {error.message}
+            </div>
+          ))}
+        </div>
+      )}
+
+      {/* Type-specific form */}
+      {node.type === 'decision' && (
+        <NodeFormDecision node={node} onUpdate={handleUpdate} />
+      )}
+      {node.type === 'action' && (
+        <NodeFormAction node={node} onUpdate={handleUpdate} />
+      )}
+      {node.type === 'solution' && (
+        <NodeFormResolution node={node} onUpdate={handleUpdate} />
+      )}
+    </Modal>
+  )
+}
+
+export default NodeEditorModal