fix: display default image if provided link is dead

Refs BIG-MAP#13

src/mod/templates/main_index.html

@@ -20,7 +20,9 @@ <h1>
             <div class="submenu-img">
                 <a href="{{ app.subpage }}">
                 {% if app.logo %}
-                    <img src="{{ app.logo }}" alt="{{ app.metadata.title }}">
+                    <!-- img with error handler for dead links (fallback to big-map logo) -->
+                    <img src="{{ app.logo }}" alt="{{ app.metadata.title }}" 
+                        onerror="this.src='static/img/big-map-white-transparent.png';this.onerror='';">
                 {% else %}
                     <img src="static/img/big-map-white-transparent.png" alt="{{ app.metadata.title }}">
                 {% endif %}