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-CN.json

@@ -647,6 +647,12 @@
     "clients": {
       "tabBasics": "基本",
       "tabCredentials": "凭据",
+      "tabLinks": "链接",
+      "linksHint": "添加第三方分享链接和远程订阅地址，将其包含在该客户端的订阅中。",
+      "addExternalLink": "添加外部链接",
+      "addExternalSubscription": "添加外部订阅",
+      "noExternalLinks": "暂无外部链接。",
+      "noExternalSubscriptions": "暂无外部订阅。",
       "add": "添加客户端",
       "edit": "编辑客户端",
       "submitAdd": "添加客户端",