feat(clients): orphan cleanup + export/import via CodeMirror modals

Add three client-management actions to the Clients page More menu:

- Delete unattached clients: removes every client with no inbound
  attachment, cascading its traffic rows, IP log, and external links
  (POST /clients/delOrphans).
- Export clients: shows the {client, inboundIds} list in a read-only
  CodeMirror viewer with copy/download (GET /clients/export returns the
  array in the standard envelope).
- Import clients: pastes that JSON into an editable CodeMirror editor,
  mirroring Import an Inbound (POST /clients/import takes a { data }
  body). Attached clients go through the create-and-attach path; items
  with no inboundIds are restored as bare records; existing emails are
  never overwritten and are reported as skipped.

Document the new endpoints in api-docs and translate the new strings
into all supported languages.

internal/web/translation/zh-TW.json

@@ -828,6 +828,12 @@
       "delDepleted": "刪除已耗盡",
       "delDepletedConfirmTitle": "刪除已耗盡的客戶端？",
       "delDepletedConfirmContent": "刪除所有流量配額已用盡或已過期的客戶端。此操作無法復原。",
+      "exportClients": "匯出客戶端",
+      "importClients": "匯入客戶端",
+      "import": "匯入",
+      "delOrphans": "刪除未關聯的客戶端",
+      "delOrphansConfirmTitle": "刪除沒有入站的客戶端？",
+      "delOrphansConfirmContent": "移除所有未關聯任何入站的客戶端，連同其流量紀錄一併刪除。此操作無法復原。",
       "auth": "認證",
       "hysteriaAuth": "Hysteria 認證",
       "uuid": "UUID",
@@ -850,7 +856,10 @@
         "bulkCreatedMixed": "已建立 {ok} 個，失敗 {failed} 個",
         "bulkAdjusted": "已調整 {count} 個客戶端",
         "bulkAdjustedMixed": "已調整 {ok} 個，跳過 {skipped} 個",
-        "delDepleted": "已刪除 {count} 個已耗盡的客戶端"
+        "delDepleted": "已刪除 {count} 個已耗盡的客戶端",
+        "delOrphans": "已刪除 {count} 個未關聯的客戶端",
+        "imported": "已匯入 {count} 個客戶端",
+        "importedMixed": "已匯入 {ok} 個，跳過 {failed} 個"
       }
     },
     "groups": {