fix(hero): wire dead "See Demo" button to sales mailto as "Book a Demo"

The hero had a <button>See Demo</button> with no handler — clicking did
nothing. Convert it to the same mailto:sales@flowercms.cloud link used by
every other CTA, with subject=Demo request so a rep can route it as a demo
booking distinct from a plain "Contact Sales" inquiry.

index.html

@@ -727,7 +727,7 @@
   <p>White-label content management for small radio and internet broadcasters. Professional websites, custom branding, and native mobile apps — all from one dashboard.</p>
   <div class="hero-cta">
     <a href="mailto:sales@flowercms.cloud" class="btn-primary">Contact Sales</a>
-    <button class="btn-secondary">See Demo</button>
+    <a href="mailto:sales@flowercms.cloud?subject=Demo%20request" class="btn-secondary">Book a Demo</a>
   </div>
 </section>