fix: propagate nmap errors to UI + sudo scan script

- scanner.py: re-raise nmap exceptions so they reach ScanRun.error
  (previously silently returned [] hiding the root cause)
- Sidebar: after triggering scan, auto-switch to History tab
- ScanHistoryPanel: auto-refresh every 3s while any run is 'running';
  show error toast when a run transitions running→error; show spinner
  on running runs; error message fully visible (no truncation)
- scripts/run_scan.py: standalone scan script to run with sudo for
  nmap OS detection / SYN scans on macOS

backend/app/services/scanner.py

@@ -28,7 +28,7 @@ def _nmap_scan(target: str) -> list[dict]:
         nm.scan(hosts=target, arguments="-sV --open -T4 --host-timeout 30s")
     except Exception as exc:
         logger.error("nmap scan failed: %s", exc)
-        return []
+        raise RuntimeError(str(exc)) from exc
 
     hosts = []
     for host in nm.all_hosts():