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/api/index.ts

@@ -17,7 +17,6 @@ export { targetListsApi } from './targetLists'
 export { maintenanceSchedulesApi, batchLaunchApi } from './maintenanceSchedules'
 export { default as feedbackApi } from './feedback'
 export { default as aiBuilderApi } from './aiBuilder'
-export { default as aiChatApi } from './aiChat'
 export { copilotApi } from './copilot'
 export { assistantChatApi } from './assistantChat'
 export { flowTransferApi } from './flowTransfer'