feat: add tunnel status indicator with control panel in header

Green pulsing dot in the desktop header shows when Cloudflare tunnel is active.
Clicking opens a dropdown panel with tunnel URL, remote client count, auth
sessions, and start/stop/QR/revoke controls. Detects tunnel clients via
Cf-Connecting-Ip header to exclude local connections from the count.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/web/public/index.html

@@ -59,6 +59,9 @@
       </div>
 
       <div class="header-right">
+        <button class="tunnel-indicator" id="tunnelIndicator" style="display: none;" onclick="app.toggleTunnelPanel()" title="Cloudflare Tunnel" aria-label="Tunnel status">
+          <span class="tunnel-dot"></span>
+        </button>
         <div class="connection-indicator" id="connectionIndicator" style="display: none;">
           <span class="connection-dot" id="connectionDot"></span>
           <span class="connection-text" id="connectionText"></span>