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/TreeEditorLayout.tsx

@@ -0,0 +1,44 @@
+import { TreeMetadataForm } from './TreeMetadataForm'
+import { NodeList } from './NodeList'
+import { TreePreviewPanel } from '@/components/tree-preview/TreePreviewPanel'
+import { cn } from '@/lib/utils'
+
+interface TreeEditorLayoutProps {
+  isMobile?: boolean
+}
+
+export function TreeEditorLayout({ isMobile = false }: TreeEditorLayoutProps) {
+  return (
+    <div
+      className={cn(
+        'flex flex-1 overflow-hidden',
+        isMobile ? 'flex-col' : 'flex-row'
+      )}
+    >
+      {/* Left Panel - Form Editor */}
+      <div
+        className={cn(
+          'flex flex-col overflow-y-auto border-border bg-background',
+          isMobile ? 'h-full w-full border-b' : 'w-3/5 border-r'
+        )}
+      >
+        <div className="space-y-4 p-4">
+          <TreeMetadataForm />
+          <NodeList />
+        </div>
+      </div>
+
+      {/* Right Panel - Preview */}
+      <div
+        className={cn(
+          'flex-1 overflow-hidden bg-muted/30',
+          isMobile ? 'hidden' : 'block'
+        )}
+      >
+        <TreePreviewPanel />
+      </div>
+    </div>
+  )
+}
+
+export default TreeEditorLayout