feat: add Script Builder page with chat UI, code blocks, preview modal, and save dialog

- ScriptCodeBlock: collapsed code preview with syntax highlighting (first 5 lines)
- ScriptBuilderInput: auto-resize chat input with Enter-to-send
- ScriptBuilderChat: message list with markdown rendering and code blocks
- ScriptPreviewModal: fullscreen script viewer with line numbers
- SaveToLibraryDialog: save script with name, description, category, team sharing
- ScriptBuilderPage: language selector, session management, FlowPilot handoff
- Added route, sidebar nav item (fuchsia), and mobile nav entry

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/layout/AppLayout.tsx

@@ -1,6 +1,6 @@
 import { useEffect, useState, useCallback } from 'react'
 import { useLocation, useNavigate, Link } from 'react-router-dom'
-import { Menu, X, LayoutGrid, Clock, Network, AlertTriangle, Code2, BarChart3, Settings, LogOut, Shield, Library } from 'lucide-react'
+import { Menu, X, LayoutGrid, Clock, Network, AlertTriangle, Code2, Wand2, BarChart3, Settings, LogOut, Shield, Library } from 'lucide-react'
 import { useAuthStore } from '@/store/authStore'
 import { usePermissions } from '@/hooks/usePermissions'
 import { useUserPreferencesStore } from '@/store/userPreferencesStore'
@@ -57,6 +57,7 @@ export function AppLayout() {
     { path: '/trees', label: 'Flows', icon: Network },
     { path: '/step-library', label: 'Step Library', icon: Library },
     { path: '/scripts', label: 'Scripts', icon: Code2 },
+    { path: '/script-builder', label: 'Script Builder', icon: Wand2 },
     { path: '/analytics', label: 'Analytics', icon: BarChart3 },
     { path: '/account', label: 'Account', icon: Settings },
   ]