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>

assets/static/index.html

@@ -53,6 +53,7 @@
             <!-- Summary stats -->
             <div class="stats-summary">
                 <span id="aircraft-count">0 aircraft</span>
+                <span id="position-summary">0 positioned</span>
                 <span id="sources-count">0 sources</span>
                 <span id="active-clients">1 viewer</span>
                 <span id="connection-status" class="connection-status disconnected">Connecting...</span>
@@ -208,6 +209,14 @@
                         <h3>Max Range</h3>
                         <div class="stat-value" id="max-range">0 km</div>
                     </div>
+                    <div class="stat-card">
+                        <h3>Aircraft with Position</h3>
+                        <div class="stat-value" id="aircraft-with-position">0</div>
+                    </div>
+                    <div class="stat-card">
+                        <h3>Aircraft without Position</h3>
+                        <div class="stat-value" id="aircraft-without-position">0</div>
+                    </div>
                 </div>
                 
                 <!-- Charts -->