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/common/CreateFlowDropdown.tsx

@@ -79,8 +79,8 @@ export function CreateFlowDropdown({
                 >
                   <Sparkles className="h-4 w-4 text-primary" />
                   <div className="text-left">
-                    <div className="font-medium">Build with AI</div>
-                    <div className="text-xs text-muted-foreground">AI-assisted flow creation</div>
+                    <div className="font-medium">Flow Assist</div>
+                    <div className="text-xs text-muted-foreground">AI-powered flow builder</div>
                   </div>
                 </button>
               </>