Fix agent mode issues and WiFi deep scan polling

Agent fixes:
- Fix Ctrl+C hang by running cleanup in background thread
- Add force-exit on double Ctrl+C
- Improve exception handling in output reader threads to prevent
  bad file descriptor errors on shutdown
- Reduce cleanup timeouts for faster shutdown

Controller/UI fixes:
- Add URL validation for agent registration (check port, protocol)
- Show helpful message when agent is unreachable during registration
- Clarify API key field label (reserved for future use)
- Add client-side URL validation with user-friendly error messages

WiFi agent mode fixes:
- Add polling fallback for deep scan when push mode is disabled
- Polls /controller/agents/{id}/wifi/data every 2 seconds
- Detect running scans when switching to an agent
- Fix scan_mode detection (agent uses params.scan_type)

templates/agents.html

@@ -337,6 +337,7 @@
                     <div class="form-group">
                         <label for="agentApiKey">API Key (optional)</label>
                         <input type="text" id="agentApiKey" placeholder="shared-secret">
+                        <small style="color: #888; font-size: 11px;">Required if agent has push mode enabled with API key</small>
                     </div>
                 </div>
                 <div class="form-row">
@@ -455,6 +456,22 @@
             const apiKey = document.getElementById('agentApiKey').value.trim();
             const description = document.getElementById('agentDescription').value.trim();
 
+            // Validate URL format
+            try {
+                const url = new URL(baseUrl);
+                if (!url.port && !baseUrl.includes(':80') && !baseUrl.includes(':443')) {
+                    showToast('URL should include a port (e.g., http://192.168.1.50:8020)', 'error');
+                    return;
+                }
+                if (url.protocol !== 'http:' && url.protocol !== 'https:') {
+                    showToast('URL must start with http:// or https://', 'error');
+                    return;
+                }
+            } catch (e) {
+                showToast('Invalid URL format. Use: http://IP_ADDRESS:PORT', 'error');
+                return;
+            }
+
             try {
                 const response = await fetch('/controller/agents', {
                     method: 'POST',