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/ja-JP.json

@@ -2075,8 +2075,5 @@
     "statusFailed": "失敗",
     "statusDown": "ダウン",
     "statusUp": "アップ"
-  },
-  "subCopyPageTitle": "サブスクリプションリンク",
-  "subCopyPageHeading": "これはサブスクリプションリンクです",
-  "subCopyPageInstructions": "ブラウザーで開く必要はありません。このページのアドレスをコピーしてアプリに貼り付けてください。"
+  }
 }