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-TW.json

@@ -747,6 +747,9 @@
       "addClients": "新增客戶端",
       "limitIp": "IP 限制",
       "limitIpDesc": "最大同時連線 IP 數。0 = 不限制。",
+      "limitIpFail2banMissing": "未安裝 Fail2ban，無法實施 IP 限制。請從 x-ui 命令列選單安裝 Fail2ban 以啟用此選項。",
+      "limitIpFail2banWindows": "Windows 上不支援 Fail2ban，無法實施 IP 限制。",
+      "limitIpDisabled": "此伺服器已停用 IP 限制功能。",
       "password": "密碼",
       "subId": "訂閱 ID",
       "online": "上線",