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)

utils/agent_client.py

@@ -141,6 +141,10 @@ class AgentClient:
         except requests.RequestException as e:
             raise AgentHTTPError(f"Request failed: {e}")
 
+    def post(self, path: str, data: dict | None = None) -> dict:
+        """Public POST method for arbitrary endpoints."""
+        return self._post(path, data)
+
     # =========================================================================
     # Capability & Status
     # =========================================================================