feat(iplimit): gate IP limit on fail2ban and reset stale limits

Per-client IP limit only enforces where fail2ban is installed, so the panel now reports enforceability and disables the field otherwise:

- Add GET /panel/api/server/fail2banStatus (enabled/installed/usable/windows), cached 30s.
- ClientFormModal and ClientBulkAddModal disable the IP Limit input when not usable and show a hover tooltip; Windows gets a platform-specific message instead of the bash-menu hint.
- One-time migration ResetIpLimitNoFail2ban zeroes existing client limitIp (inbound settings JSON + clients table) on hosts without fail2ban, where the limit never applied.
- Drop the recurring '[LimitIP] Fail2Ban is not installed' warning.
- Add limitIpFail2banMissing/limitIpFail2banWindows/limitIpDisabled across all 13 locales.

frontend/src/api/queries/useFail2banStatusQuery.ts

@@ -0,0 +1,41 @@
+import { useQuery } from '@tanstack/react-query';
+
+import { HttpUtil } from '@/utils';
+import { keys } from '@/api/queryKeys';
+
+export interface Fail2banStatus {
+  enabled: boolean;
+  installed: boolean;
+  usable: boolean;
+  windows: boolean;
+}
+
+const FAIL_OPEN_STATUS: Fail2banStatus = {
+  enabled: true,
+  installed: true,
+  usable: true,
+  windows: false,
+};
+
+async function fetchFail2banStatus(): Promise<Fail2banStatus> {
+  const msg = await HttpUtil.get<Fail2banStatus>('/panel/api/server/fail2banStatus', undefined, { silent: true });
+  if (!msg?.success || !msg.obj) throw new Error(msg?.msg || 'Failed to fetch fail2ban status');
+  return { ...FAIL_OPEN_STATUS, ...msg.obj };
+}
+
+export function getLimitIpNotice(status: Fail2banStatus, t: (key: string) => string): string {
+  if (status.usable) return '';
+  if (!status.enabled) return t('pages.clients.limitIpDisabled');
+  if (status.windows) return t('pages.clients.limitIpFail2banWindows');
+  return t('pages.clients.limitIpFail2banMissing');
+}
+
+export function useFail2banStatusQuery() {
+  const query = useQuery({
+    queryKey: keys.server.fail2banStatus(),
+    queryFn: fetchFail2banStatus,
+    staleTime: 60_000,
+  });
+
+  return query.data ?? FAIL_OPEN_STATUS;
+}

frontend/src/api/queryKeys.ts

@@ -1,6 +1,7 @@
 export const keys = {
   server: {
     status: () => ['server', 'status'] as const,
+    fail2banStatus: () => ['server', 'fail2banStatus'] as const,
   },
   nodes: {
     root: () => ['nodes'] as const,