feat(sub): per-client external links and remote subscriptions

Add a Links tab to the client form for attaching third-party share
links and remote subscription URLs per client. They are merged into
the client's raw/JSON/Clash subscription output: links are emitted
verbatim and parsed for JSON/Clash; subscription URLs are fetched
(cached, with a short timeout) and their configs merged in.

i18n keys added across all 13 locales.

internal/web/translation/zh-TW.json

@@ -647,6 +647,12 @@
     "clients": {
       "tabBasics": "基本",
       "tabCredentials": "認證資訊",
+      "tabLinks": "連結",
+      "linksHint": "新增第三方分享連結和遠端訂閱網址，將其包含在此用戶端的訂閱中。",
+      "addExternalLink": "新增外部連結",
+      "addExternalSubscription": "新增外部訂閱",
+      "noExternalLinks": "尚無外部連結。",
+      "noExternalSubscriptions": "尚無外部訂閱。",
       "add": "新增客戶端",
       "edit": "編輯客戶端",
       "submitAdd": "新增客戶端",