Add contact form to footer and simplify footer content

Replace footer note with a contact form that emails contact@schoolcompare.co.uk
via FormSubmit.co. Keep only the data source attribution. Update CSP to allow
form submissions to FormSubmit.co and add responsive styling for the form.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/app.py

@@ -72,7 +72,7 @@ class SecurityHeadersMiddleware(BaseHTTPMiddleware):
             "connect-src 'self' https://cdn.jsdelivr.net https://*.tile.openstreetmap.org https://unpkg.com https://www.google-analytics.com https://analytics.google.com https://*.google-analytics.com; "
             "frame-ancestors 'none'; "
             "base-uri 'self'; "
-            "form-action 'self';"
+            "form-action 'self' https://formsubmit.co;"
         )
 
         # HSTS (only enable if using HTTPS in production)