fix(sub): serve a copy-only page when a subscription URL is opened in a browser (#6183)

* fix(sub): show copy-only page for browser subscription visits

Browser navigation to /sub previously rendered the normal subscription page, which exposed subscription material in page data or raw base64 depending on request headers. Keep VPN clients on the raw subscription body, but classify browser document requests and return a neutral static copy-only HTML page with no embedded share links or page data.

This preserves the C1 LimitIP parser fix in the same master candidate while avoiding a DE rollback of the browser subscription UX.

* fix(sub): keep the themed page for an explicit html request

Only implicit browser navigation is downgraded to the copy-only page. An
operator who appends html=1 or view=html already holds the URL, so the
themed subscription page keeps rendering for them and serveSubPage stays
in use.

* fix(sub): keep browser pages copy-only

internal/web/translation/ru-RU.json

@@ -2075,5 +2075,8 @@
     "statusFailed": "НЕУДАЧНО",
     "statusDown": "НЕДОСТУПЕН",
     "statusUp": "РАБОТАЕТ"
-  }
+  },
+  "subCopyPageTitle": "Ссылка подписки",
+  "subCopyPageHeading": "Это ссылка подписки",
+  "subCopyPageInstructions": "Открывать её в браузере не нужно. Скопируйте адрес этой страницы и вставьте его в приложение."
 }