refactor: remove standalone Flow Assist page and old AI chat components

Remove the old /ai/chat page, AI wizard modal, and all associated
components/stores/types now replaced by the editor-embedded AI panel.

Deleted:
- AIChatBuilderPage, ai-chat/ components, aiChatStore, aiChat API, ai-chat types
- AIFlowBuilderModal, ai-builder/ components, aiFlowBuilderStore

Cleaned up:
- Router (removed /ai/chat route)
- Sidebar (removed Flow Assist nav item)
- MyTreesPage (removed AI builder modal and button)
- TreeLibraryPage (removed Flow Assist button)
- API and type barrel exports

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/pages/TreeLibraryPage.tsx

@@ -1,6 +1,6 @@
 import { useEffect, useState, useCallback, useMemo } from 'react'
 import { useNavigate, useSearchParams } from 'react-router-dom'
-import { X, RotateCcw, Play, Sparkles, FileUp } from 'lucide-react'
+import { X, RotateCcw, Play, FileUp } from 'lucide-react'
 import { treesApi } from '@/api/trees'
 import { categoriesApi } from '@/api/categories'
 import { foldersApi } from '@/api/folders'
@@ -283,15 +283,6 @@ export function TreeLibraryPage() {
             </div>
             {canCreateTrees && (
               <div className="flex items-center gap-2">
-                {aiEnabled && (
-                  <button
-                    onClick={() => navigate(typeFilter === 'procedural' || typeFilter === 'maintenance' ? `/ai/chat?type=${typeFilter}` : '/ai/chat')}
-                    className="flex items-center gap-2 rounded-lg bg-gradient-brand px-4 py-2 text-sm font-medium text-white shadow-lg shadow-primary/20 hover:opacity-90 transition-opacity"
-                  >
-                    <Sparkles className="h-4 w-4" />
-                    Flow Assist
-                  </button>
-                )}
                 <button
                   onClick={() => setShowImportModal(true)}
                   className="flex items-center gap-2 rounded-lg border border-border bg-[rgba(255,255,255,0.04)] px-4 py-2 text-sm font-medium text-foreground hover:border-[rgba(255,255,255,0.12)] transition-colors"