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/pages/TreeLibraryPage.tsx

@@ -279,7 +279,7 @@ export function TreeLibraryPage() {
                     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" />
-                    Build with AI
+                    Flow Assist
                   </button>
                 )}
                 <CreateFlowDropdown