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/en-US.json

@@ -648,6 +648,12 @@
     "clients": {
       "tabBasics": "Basics",
       "tabCredentials": "Credentials",
+      "tabLinks": "Links",
+      "linksHint": "Add third-party share links and remote subscription URLs to include in this client's subscription.",
+      "addExternalLink": "Add External Link",
+      "addExternalSubscription": "Add External Subscription",
+      "noExternalLinks": "No external links yet.",
+      "noExternalSubscriptions": "No external subscriptions yet.",
       "add": "Add Client",
       "edit": "Edit Client",
       "submitAdd": "Add Client",
@@ -1765,4 +1771,4 @@
       "chooseInbound": "Choose an Inbound"
     }
   }
-}
+}