fix: preserve rotation logs during quit fallback

Route synchronous account-rotation shutdown flushes through the internal log writer so failures under userData/logs retry userData/internal-logs in the same order as runtime writes.

Keep buffered lines until a target accepts the full chunk, report the successful fallback as the active path, and retain the buffer when every internal target fails.

Cover the real EISDIR primary-path case, complete fallback failure, and main-process quit delegation without launching Electron.

main.js

@@ -1785,8 +1785,7 @@ app.on('will-quit', () => {
   } catch {}
   try {
     if (_rotLogBuffer.length) {
-      fs.appendFileSync(getRotLogPath(), _rotLogBuffer.join(''), 'utf-8');
-      _rotLogBuffer.length = 0;
+      _rotLogWriter.flushSync(_rotLogBuffer, 'rot-log');
     }
   } catch {}
 });