Narrow jail config types with explicit union values

frontend/src/components/config/JailSectionPanel.tsx

@@ -12,7 +12,7 @@ import {
 import { KVEditor } from "./KVEditor";
 import { StringListEditor } from "./StringListEditor";
 import { useConfigStyles } from "./configStyles";
-import type { JailSectionConfig } from "../../types/config";
+import type { BackendType, JailSectionConfig } from "../../types/config";
 
 const BACKENDS = ["", "auto", "polling", "gamin", "pyinotify", "systemd"] as const;
 
@@ -64,7 +64,9 @@ export function JailSectionPanel({ jailName, section, onChange }: JailSectionPan
             <Select
               size="small"
               value={section.backend ?? ""}
-              onChange={(_e, d) => { update({ backend: d.value || null }); }}
+              onChange={(_e, d) => {
+                update({ backend: d.value ? (d.value as BackendType) : null });
+              }}
             >
               {BACKENDS.map((b) => (
                 <option key={b} value={b}>