feat(frontend): implement center auth session flow with route guard

src/auth/ProtectedRoute.tsx

@@ -0,0 +1,34 @@
+import { Navigate } from 'react-router-dom'
+import { useAuth } from './auth-context'
+import type { PropsWithChildren } from 'react'
+import { useEffect, useState } from 'react'
+
+export default function ProtectedRoute({ children }: PropsWithChildren) {
+  const { isAuthed, ensureFreshToken, logout } = useAuth()
+  const [ready, setReady] = useState(false)
+
+  useEffect(() => {
+    let active = true
+    ;(async () => {
+      if (!isAuthed) {
+        if (active) setReady(true)
+        return
+      }
+      try {
+        await ensureFreshToken()
+      } catch {
+        await logout()
+      } finally {
+        if (active) setReady(true)
+      }
+    })()
+
+    return () => {
+      active = false
+    }
+  }, [isAuthed, ensureFreshToken, logout])
+
+  if (!ready) return null
+  if (!isAuthed) return <Navigate to="/login" replace />
+  return <>{children}</>
+}