fix(sub): restore the subscription info page for browser visits

Revert 43bc9153 and its follow-up 338822ab. The copy-only notice replaced the
themed sub page for every browser request, so mobile users got a bare "This is
a subscription link" screen instead of their traffic, expiry and links — and it
left serveSubPage plus the custom-theme renderer as dead code.

internal/web/translation/zh-TW.json

@@ -2075,8 +2075,5 @@
     "statusFailed": "失敗",
     "statusDown": "中斷",
     "statusUp": "恢復"
-  },
-  "subCopyPageTitle": "訂閱連結",
-  "subCopyPageHeading": "這是訂閱連結",
-  "subCopyPageInstructions": "無需在瀏覽器中開啟。請複製此頁面位址並貼到應用程式中。"
+  }
 }