feat: rename AI builder to "Flow Assist" and add sidebar nav item

- Rename all "Build with AI" labels to "Flow Assist"
- Update CreateFlowDropdown description to "AI-powered flow builder"
- Add Flow Assist nav item with Sparkles icon to sidebar (both
  expanded and collapsed states)
- Update AIFlowBuilderModal titles for consistency

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

frontend/src/components/ai-builder/AIFlowBuilderModal.tsx

@@ -98,7 +98,7 @@ export function AIFlowBuilderModal({ isOpen, onClose }: AIFlowBuilderModalProps)
   const getTitle = () => {
     switch (phase) {
       case 'foundation':
-        return 'Build with AI'
+        return 'Flow Assist'
       case 'scaffolding':
       case 'generating':
         return 'AI Scaffold'
@@ -107,9 +107,9 @@ export function AIFlowBuilderModal({ isOpen, onClose }: AIFlowBuilderModalProps)
       case 'reviewing':
         return 'Review & Assemble'
       case 'error':
-        return 'AI Flow Builder'
+        return 'Flow Assist'
       default:
-        return 'Build with AI'
+        return 'Flow Assist'
     }
   }