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.

internal/web/translation/zh-CN.json

@@ -747,6 +747,9 @@
       "addClients": "添加客户端",
       "limitIp": "IP 限制",
       "limitIpDesc": "最大同时连接 IP 数。0 = 不限制。",
+      "limitIpFail2banMissing": "未安装 Fail2ban，无法实施 IP 限制。请从 x-ui 命令行菜单安装 Fail2ban 以启用此选项。",
+      "limitIpFail2banWindows": "Windows 上不支持 Fail2ban，无法实施 IP 限制。",
+      "limitIpDisabled": "此服务器已禁用 IP 限制功能。",
       "password": "密码",
       "subId": "订阅 ID",
       "online": "在线",