fix: PR review fixes — rename overlay, remove dead code, fix height measurement

- Rename save-template-overlay.tsx to export-overlay.tsx to match component name
- Remove unused demo-gallery components (preserved on feat/demo-gallery branch)
- Fix widget height measurement using offscreen clone instead of collapsing
  visible DOM, preventing potential ResizeObserver infinite loops
- Use camelCase keyElements prop in PlanCard (remap from snake_case at call site)
- Update chat disclaimer to be relevant to visualization demo

Author: GeneralJerel

apps/app/src/app/page.tsx

@@ -76,6 +76,7 @@ export default function HomePage() {
             <CopilotChat
               labels={{
                 welcomeMessageText: "What do you want to visualize today?",
+                chatDisclaimerText: "Visualizations are AI-generated. You can retry the same prompt or ask the AI to refine the result.",
               }}
             />
           } />