Add active WebSocket client tracking and display

Track and display the number of active WebSocket connections viewing the
aircraft tracker. Features:

Backend:
- Add getActiveClientCount() method for thread-safe client counting
- Include active_clients count in all statistics responses
- Integrate with existing stats API endpoints

Frontend:
- Display "X viewer(s)" in header stats with proper plural/singular grammar
- Add "Active Viewers" card to Statistics view
- Real-time updates as clients connect/disconnect
- Fallback to 1 when stats not yet available

This adds a social element showing "Now being watched by X users" style
information prominently in the interface.

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

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

assets/static/js/modules/ui-manager.js

@@ -241,6 +241,7 @@ export class UIManager {
     updateStatistics() {
         const totalAircraftEl = document.getElementById('total-aircraft');
         const activeSourcesEl = document.getElementById('active-sources');
+        const activeViewersEl = document.getElementById('active-viewers');
         const maxRangeEl = document.getElementById('max-range');
         const messagesSecEl = document.getElementById('messages-sec');
         
@@ -248,6 +249,9 @@ export class UIManager {
         if (activeSourcesEl) {
             activeSourcesEl.textContent = Array.from(this.sourcesData.values()).filter(s => s.active).length;
         }
+        if (activeViewersEl) {
+            activeViewersEl.textContent = this.stats.active_clients || 1;
+        }
         
         // Calculate max range
         let maxDistance = 0;
@@ -267,10 +271,18 @@ export class UIManager {
     updateHeaderInfo() {
         const aircraftCountEl = document.getElementById('aircraft-count');
         const sourcesCountEl = document.getElementById('sources-count');
+        const activeClientsEl = document.getElementById('active-clients');
         
         if (aircraftCountEl) aircraftCountEl.textContent = `${this.aircraftData.size} aircraft`;
         if (sourcesCountEl) sourcesCountEl.textContent = `${this.sourcesData.size} sources`;
         
+        // Update active clients count
+        const clientCount = this.stats.active_clients || 1;
+        if (activeClientsEl) {
+            const clientText = clientCount === 1 ? 'viewer' : 'viewers';
+            activeClientsEl.textContent = `${clientCount} ${clientText}`;
+        }
+        
         this.updateClocks();
     }