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).

app/(main)/[locale]/page.tsx

@@ -3101,11 +3101,13 @@ export default function Home() {
                     onForward={handleForward}
                     onDelete={() => {
                       // Deleting the open message returns to the list (Gmail-style),
-                      // not the next email. Deselect first so the store's
-                      // remove-and-advance sees no selection and doesn't auto-open
-                      // the next message; then delete the captured email.
+                      // not the next email — unless the user turned the setting off.
+                      // Deselect first so the store's remove-and-advance sees no
+                      // selection and doesn't auto-open the next message.
                       const target = selectedEmail;
-                      handleMobileBack();
+                      if (useSettingsStore.getState().returnToListAfterAction) {
+                        handleMobileBack();
+                      }
                       handleDelete(target);
                     }}
                     onArchive={() => handleArchive()}