feat: allow interfaces to have no zone (shorewall '-' zone)

frontend/src/routes/ConfigDetail.tsx

@@ -83,13 +83,13 @@ export default function ConfigDetail() {
         {
           key: 'zone_id' as const,
           label: 'Zone',
-          render: (r: AnyEntity) => zones.find((z) => z.id === r['zone_id'])?.name ?? String(r['zone_id']),
+          render: (r: AnyEntity) => r['zone_id'] == null ? '-' : (zones.find((z) => z.id === r['zone_id'])?.name ?? String(r['zone_id'])),
         },
         { key: 'options' as const, label: 'Options' },
       ] as Column<AnyEntity>[],
       fields: [
         { name: 'name', label: 'Interface Name', required: true },
-        { name: 'zone_id', label: 'Zone', required: true, type: 'select' as const, options: zoneOptions },
+        { name: 'zone_id', label: 'Zone', type: 'select' as const, options: [{ value: '', label: '- (no zone)' }, ...zoneOptions] },
         { name: 'options', label: 'Options' },
       ] as FieldDef[],
     },