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/common/Modal.tsx

@@ -0,0 +1,101 @@
+import { useEffect, useCallback, type ReactNode } from 'react'
+import { X } from 'lucide-react'
+import { cn } from '@/lib/utils'
+
+interface ModalProps {
+  isOpen: boolean
+  onClose: () => void
+  title: string
+  children: ReactNode
+  /** Optional footer content that stays fixed at bottom (doesn't scroll) */
+  footer?: ReactNode
+  size?: 'sm' | 'md' | 'lg' | 'xl'
+}
+
+export function Modal({ isOpen, onClose, title, children, footer, size = 'md' }: ModalProps) {
+  // Close on Escape key
+  const handleKeyDown = useCallback(
+    (e: KeyboardEvent) => {
+      if (e.key === 'Escape') {
+        onClose()
+      }
+    },
+    [onClose]
+  )
+
+  useEffect(() => {
+    if (isOpen) {
+      document.addEventListener('keydown', handleKeyDown)
+      document.body.style.overflow = 'hidden'
+    }
+    return () => {
+      document.removeEventListener('keydown', handleKeyDown)
+      document.body.style.overflow = ''
+    }
+  }, [isOpen, handleKeyDown])
+
+  if (!isOpen) return null
+
+  const sizeClasses = {
+    sm: 'max-w-sm',
+    md: 'max-w-md',
+    lg: 'max-w-lg',
+    xl: 'max-w-4xl',
+  }
+
+  return (
+    <div
+      className="fixed inset-0 z-50 flex items-center justify-center p-4"
+      role="dialog"
+      aria-modal="true"
+      aria-labelledby="modal-title"
+    >
+      {/* Backdrop */}
+      <div
+        className="absolute inset-0 bg-background/80 backdrop-blur-sm"
+        onClick={onClose}
+        aria-hidden="true"
+      />
+
+      {/* Modal Content */}
+      <div
+        className={cn(
+          'relative flex max-h-[85vh] w-full flex-col rounded-lg border border-border bg-card shadow-lg',
+          sizeClasses[size]
+        )}
+      >
+        {/* Header - Fixed at top */}
+        <div className="flex flex-shrink-0 items-center justify-between border-b border-border px-6 py-4">
+          <h2 id="modal-title" className="text-lg font-semibold text-card-foreground">
+            {title}
+          </h2>
+          <button
+            onClick={onClose}
+            className={cn(
+              'rounded-md p-1 text-muted-foreground transition-colors',
+              'hover:bg-accent hover:text-accent-foreground',
+              'focus:outline-none focus:ring-2 focus:ring-ring'
+            )}
+            aria-label="Close modal"
+          >
+            <X className="h-5 w-5" />
+          </button>
+        </div>
+
+        {/* Body - Scrollable */}
+        <div className="flex-1 overflow-y-auto px-6 py-4">
+          {children}
+        </div>
+
+        {/* Footer - Fixed at bottom */}
+        {footer && (
+          <div className="flex-shrink-0 border-t border-border px-6 py-4">
+            {footer}
+          </div>
+        )}
+      </div>
+    </div>
+  )
+}
+
+export default Modal