Fix issue #21 and add aircraft position tracking indicators

- Fix Debian package upgrade issue by separating upgrade vs remove behavior in prerm script
- Add aircraft position tracking statistics in merger GetStatistics() method
- Update frontend to display position tracking indicators in both header and stats view
- Format Go code to maintain consistency

🤖 Generated with [Claude Code](https://claude.ai/code)

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

debian/DEBIAN/prerm

@@ -2,8 +2,8 @@
 set -e
 
 case "$1" in
-    remove|upgrade|deconfigure)
-        # Stop and disable the service
+    remove|deconfigure)
+        # Stop and disable the service on removal
         if systemctl is-active --quiet skyview-adsb.service; then
             systemctl stop skyview-adsb.service
         fi
@@ -12,6 +12,13 @@ case "$1" in
             systemctl disable skyview-adsb.service
         fi
         ;;
+    upgrade)
+        # Only stop service during upgrade, preserve enabled state
+        if systemctl is-active --quiet skyview-adsb.service; then
+            systemctl stop skyview-adsb.service
+        fi
+        # Don't disable - postinst will restart if service was enabled
+        ;;
 esac
 
 exit 0