fix(dashboard): show skeleton instead of default layout until hydration completes

Root cause: useDashboardLayout initialised React state with createDefaultDashboardLayout()
(1 widget), so the wrong default rendered during the ~100–500ms window while React Query
fetched the user session and DB layout after login. On reload within staleTime the cache
hit resolved instantly, masking the bug.

Fix: add isHydrated boolean state that becomes true only once localStorage OR DB
hydration has settled; DashboardClient renders a GridLayoutSkeleton until then.
Also adds router.refresh() in the sign-in handler to bust the Next.js Router Cache
so the post-login navigation always lands on a fresh server component tree.

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

apps/web/src/app/auth/signin/page.tsx

@@ -59,6 +59,9 @@ export default function SignInPage() {
         setTotp("");
       }
     } else {
+      // Invalidate the Next.js Router Cache so (app)/layout.tsx re-renders
+      // with the fresh session, then navigate to the dashboard.
+      router.refresh();
       router.push("/dashboard");
     }