fix(single-instance): quit when the lock is lost

requestSingleInstanceLock() without a quit on failure means the second
process falls through to whenReady() and opens a duplicate window (what
Avi saw: two Folios from one icon click). Now the loser forwards to the
running window (second-instance) and exits. Pairs with the launcher
TMPDIR pin — uwsm per-scope /tmp made the singleton socket invisible, so
the lock never even failed before.

src/main/index.ts

@@ -1423,4 +1423,8 @@ if (app.requestSingleInstanceLock()) {
     }
     if (f) openFromArg(f);
   });
+} else {
+  // Lost the lock: another Folio is already running. Without this quit the
+  // second process falls through to whenReady() and opens a duplicate window.
+  app.quit();
 }