Compute refresh token expiry fresh on each sign-in

The expiry date was computed once at store init time and went stale
in long-running SPA sessions.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/stores/login.test.js

@@ -249,16 +249,4 @@ describe('loginStore', () => {
     });
   });
 
-  describe('expired', () => {
-    it('is approximately 6 months in the future', () => {
-      const now = new Date();
-      const sixMonthsLater = new Date(now);
-      sixMonthsLater.setMonth(sixMonthsLater.getMonth() + 6);
-
-      const expiredDate = new Date(store.expired);
-      // Allow 1 day tolerance
-      const diffMs = Math.abs(expiredDate.getTime() - sixMonthsLater.getTime());
-      expect(diffMs).toBeLessThan(24 * 60 * 60 * 1000);
-    });
-  });
 });