Point share page CTA button to boxviewer.app

Replace GitHub repo link on the main action button with the new
product website. Footer 'Source Code' link remains on GitHub.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: nichu42

share/index.html

@@ -325,7 +325,7 @@ <h1>BoxViewer for openSenseMap</h1>
 
             <p id="status-text">BoxViewer is a beautiful, tracking-free Android companion client. Loading shared senseBox station...</p>
 
-            <a href="https://github.com/nichu42/boxviewer" class="btn" id="action-btn">Get BoxViewer on GitHub</a>
+            <a href="https://boxviewer.app" class="btn" id="action-btn">Get BoxViewer</a>
 
             <footer class="footer">
                 <div class="footer-links">