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/ar-EG.json

@@ -647,6 +647,12 @@
     "clients": {
       "tabBasics": "أساسي",
       "tabCredentials": "بيانات الاعتماد",
+      "tabLinks": "الروابط",
+      "linksHint": "أضف روابط مشاركة من جهات خارجية وعناوين اشتراك خارجية لتضمينها في اشتراك هذا العميل.",
+      "addExternalLink": "إضافة رابط خارجي",
+      "addExternalSubscription": "إضافة اشتراك خارجي",
+      "noExternalLinks": "لا توجد روابط خارجية بعد.",
+      "noExternalSubscriptions": "لا توجد اشتراكات خارجية بعد.",
       "add": "إضافة عميل",
       "edit": "تعديل العميل",
       "submitAdd": "إضافة عميل",