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).

components/settings/reading-settings.tsx

@@ -22,6 +22,7 @@ export function ReadingSettings() {
     markAsReadDelay,
     deleteAction,
     permanentlyDeleteJunk,
+    returnToListAfterAction,
     showPreview,
     mailLayout,
     disableThreading,
@@ -176,6 +177,13 @@ export function ReadingSettings() {
         />
       </SettingItem>
 
+      <SettingItem label={t('return_to_list_after_action.label')} description={t('return_to_list_after_action.description')}>
+        <ToggleSwitch
+          checked={returnToListAfterAction}
+          onChange={(checked) => updateSetting('returnToListAfterAction', checked)}
+        />
+      </SettingItem>
+
       {!isSettingHidden('showPreview') && (
       <SettingItem
         label={t('show_preview.label')}