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

scripts/run_scan.py

@@ -0,0 +1,50 @@
+#!/usr/bin/env python3
+"""
+Standalone scan script — run with sudo to allow nmap OS detection and SYN scans.
+
+Usage (from the backend/ directory):
+    sudo ../scripts/run_scan.py 192.168.1.0/24
+    sudo ../scripts/run_scan.py 192.168.1.0/24 10.0.0.0/24
+
+Results are written directly to the database and appear as Pending Devices
+in the Homelable UI. The backend does not need to be restarted.
+"""
+import asyncio
+import sys
+from pathlib import Path
+
+# Make sure app/ is importable
+sys.path.insert(0, str(Path(__file__).parent.parent / "backend"))
+
+from app.db.database import AsyncSessionLocal, init_db
+from app.db.models import ScanRun
+from app.services.scanner import run_scan
+
+
+async def main(ranges: list[str]) -> None:
+    await init_db()
+    async with AsyncSessionLocal() as db:
+        run = ScanRun(status="running", ranges=ranges)
+        db.add(run)
+        await db.commit()
+        await db.refresh(run)
+        print(f"Scan started (id={run.id}) for ranges: {', '.join(ranges)}")
+
+    await run_scan(ranges, db, run.id)
+
+    async with AsyncSessionLocal() as db:
+        run = await db.get(ScanRun, run.id)
+        if run:
+            print(f"Scan {run.status} — {run.devices_found} device(s) found")
+            if run.error:
+                print(f"Error: {run.error}", file=sys.stderr)
+
+
+if __name__ == "__main__":
+    if len(sys.argv) < 2:
+        print("Usage: sudo python scripts/run_scan.py <cidr> [<cidr> ...]")
+        print("Example: sudo python scripts/run_scan.py 192.168.1.0/24")
+        sys.exit(1)
+
+    ranges = sys.argv[1:]
+    asyncio.run(main(ranges))