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-chat/ChatToolbar.tsx

@@ -27,7 +27,7 @@ export function ChatToolbar({
       <div className="flex items-center gap-3">
         <div className="flex items-center gap-2 text-sm font-medium text-foreground">
           <Sparkles className="h-4 w-4 text-primary" />
-          Build with AI
+          Flow Assist
         </div>
         <PhaseIndicator currentPhase={currentPhase} />
       </div>