Site audit fixes: metadata, download URLs, nav dropdown, CTAs, related resources

- Add metadata and h1 to homepage
- Replace all placeholder download URLs with real links
- Convert Platform nav item to dropdown with product sub-links
- Add hero CTA buttons to rover page
- Add bottom CTA section to platform page
- Add RelatedResources to junior and python pages
- Add metadata description to contact and download pages
- Update "Visit Store" to "Build Your Kit" for consistency
- Add Airtable TODO comment in contact form handler
- Update Micromelon-Py card image to VS Code screenshot

src/app/contact/ContactForm.tsx

@@ -28,7 +28,7 @@ export default function ContactForm() {
 
   const handleSubmit = async (e: React.FormEvent) => {
     e.preventDefault();
-    // POST to /api/contact will be built later
+    // TODO: Replace with Airtable form embed once URL is available
     setSubmitted(true);
   };