feat(mail): return to the list after marking an open message unread

Gmail-style: marking the currently-open message unread returns to the message
list instead of staying in the reading pane (where the viewer's auto-mark-read
would just flip it back to read). Gated on the returnToListAfterAction setting
added in #477 (default on); when off, you stay in the viewer.

Only the single-message viewer, and only on mark-unread (read === false).

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

@@ -3118,6 +3118,12 @@ export default function Home() {
                     onMarkAsRead={async (emailId, read) => {
                       if (client) {
                         await markAsRead(client, emailId, read);
+                        // Marking the open message unread returns to the list
+                        // (Gmail-style, gated on returnToListAfterAction). Staying
+                        // in the reading pane would just re-mark it read on view.
+                        if (!read && useSettingsStore.getState().returnToListAfterAction) {
+                          handleMobileBack();
+                        }
                       }
                     }}
                     onDownloadAttachment={handleDownloadAttachment}