feat(settings): make return-to-list-after-action configurable (default on)

Per review: gate the return-to-list behaviour behind a setting,
returnToListAfterAction, defaulting to true (the Gmail/Yahoo default). When off,
deleting the open message keeps the previous auto-advance-to-next behaviour.

Adds the setting to the store (persisted), a toggle under Reading settings, and
i18n keys across all locales (English; non-English need translation). The same
setting will govern mark-as-unread (#468).

locales/pt/common.json

@@ -1226,6 +1226,10 @@
         "off": "Desativado",
         "seconds": "{seconds} segundos",
         "unsupported": "A conta atual não anuncia suporte a envio atrasado. A configuração continuará salva para outras contas."
+      },
+      "return_to_list_after_action": {
+        "label": "Return to list after delete or mark unread",
+        "description": "After deleting or marking the open message unread, go back to the message list instead of opening the next message."
       }
     },
     "composer": {