🗓️ Fix date display in Recent Trees - use created_at field

- Changed tree.timestamp to tree.created_at to match Supabase schema
- This fixes the 'Invalid Date' display in Recent Trees section
- Date formatting will now work correctly for all trees

The Supabase database returns 'created_at' but frontend was looking for 'timestamp'.

static/app.js

@@ -443,7 +443,7 @@ class TreeTrackApp {
                         ${tree.scientific_name || tree.common_name || tree.local_name || 'Unnamed'}
                         <br> ${tree.latitude.toFixed(4)}, ${tree.longitude.toFixed(4)}
                         ${tree.tree_code ? '<br> ' + tree.tree_code : ''}
-                        <br> ${new Date(tree.timestamp).toLocaleDateString()}
+                        <br> ${new Date(tree.created_at).toLocaleDateString()}
                     </div>
                 </div>
             `).join('');