feat: add sign-in flow, routing shell, and searchable book list

Pin @testing-library/jest-dom to ~6.9.1 (6.10.0 is npm-deprecated).

Claude-Session: https://claude.ai/code/session_01M9MLit5Ko3X4s7rzC5Kv7X

src/auth/RequireAuth.tsx

@@ -0,0 +1,10 @@
+import { Navigate, Outlet, useLocation } from 'react-router'
+import { useAuth } from './AuthContext'
+
+export function RequireAuth() {
+  const { user, loading } = useAuth()
+  const location = useLocation()
+  if (loading) return <p className="p-6 text-slate-500">Loading…</p>
+  if (!user) return <Navigate to="/login" replace state={{ from: location }} />
+  return <Outlet />
+}