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

src/lib/auth-storage.ts

@@ -0,0 +1,47 @@
+export type AuthSession = {
+  accessToken: string
+  refreshToken: string
+  tokenType: string
+  user: {
+    id: string
+    email: string
+  }
+}
+
+const KEY = 'fabric.auth.session.v1'
+
+export function getAuthSession(): AuthSession | null {
+  const raw = localStorage.getItem(KEY)
+  if (!raw) return null
+  try {
+    return JSON.parse(raw) as AuthSession
+  } catch {
+    return null
+  }
+}
+
+export function setAuthSession(session: AuthSession): void {
+  localStorage.setItem(KEY, JSON.stringify(session))
+}
+
+export function clearAuthSession(): void {
+  localStorage.removeItem(KEY)
+}
+
+function parseJwtExpMs(token: string): number | null {
+  const payload = token.split('.')[1]
+  if (!payload) return null
+  try {
+    const decoded = JSON.parse(atob(payload)) as { exp?: number }
+    if (!decoded.exp) return null
+    return decoded.exp * 1000
+  } catch {
+    return null
+  }
+}
+
+export function isAccessTokenStale(token: string, leadMs = 60_000): boolean {
+  const expMs = parseJwtExpMs(token)
+  if (!expMs) return true
+  return Date.now() + leadMs >= expMs
+}