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/pages/api-docs/endpoints.ts

@@ -252,6 +252,12 @@ export const sections: readonly Section[] = [
         summary: 'Real-time machine snapshot: CPU, memory, swap, disk, network IO, load averages, open connections, Xray state. Cached and refreshed every 2 seconds in the background.',
         response: '{\n  "success": true,\n  "obj": {\n    "cpu": 12.5,\n    "mem": { "current": 2147483648, "total": 8589934592 },\n    "swap": { "current": 0, "total": 4294967296 },\n    "disk": { "current": 53687091200, "total": 268435456000 },\n    "netIO": { "up": 1073741824, "down": 2147483648 },\n    "xray": { "state": "running", "version": "v25.10.31" },\n    "tcpCount": 42,\n    "load": { "load1": 0.5, "load5": 0.3, "load15": 0.2 }\n  }\n}',
       },
+      {
+        method: 'GET',
+        path: '/panel/api/server/fail2banStatus',
+        summary: 'Reports whether per-client IP limits can be enforced on this host. The panel uses it to gate the "IP Limit" field, since enforcement depends on Fail2ban being installed.',
+        response: '{\n  "success": true,\n  "obj": {\n    "enabled": true,\n    "installed": true,\n    "usable": true,\n    "windows": false\n  }\n}',
+      },
       {
         method: 'GET',
         path: '/panel/api/server/cpuHistory/:bucket',