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/index.html

@@ -54,6 +54,7 @@
             <div class="stats-summary">
                 <span id="aircraft-count">0 aircraft</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>
             </div>
         </header>
@@ -195,6 +196,10 @@
                         <h3>Active Sources</h3>
                         <div class="stat-value" id="active-sources">0</div>
                     </div>
+                    <div class="stat-card">
+                        <h3>Active Viewers</h3>
+                        <div class="stat-value" id="active-viewers">1</div>
+                    </div>
                     <div class="stat-card">
                         <h3>Messages/sec</h3>
                         <div class="stat-value" id="messages-sec">0</div>