feat: implement login functionality with error handling and session management

Frontend/erp-system/lib/auth/session.ts

@@ -0,0 +1,39 @@
+import type { AuthUser, LoginResult } from "@/lib/api/auth"
+
+const ACCESS_TOKEN_KEY = "hexa_erp_access_token"
+const REFRESH_TOKEN_KEY = "hexa_erp_refresh_token"
+const USER_KEY = "hexa_erp_user"
+
+/** remember=true persists across browser restarts (localStorage); otherwise session-only. */
+export function persistSession(result: LoginResult, remember: boolean) {
+  const store = remember ? window.localStorage : window.sessionStorage
+  const other = remember ? window.sessionStorage : window.localStorage
+  for (const key of [ACCESS_TOKEN_KEY, REFRESH_TOKEN_KEY, USER_KEY]) other.removeItem(key)
+
+  store.setItem(ACCESS_TOKEN_KEY, result.accessToken)
+  store.setItem(REFRESH_TOKEN_KEY, result.refreshToken)
+  store.setItem(USER_KEY, JSON.stringify(result.user))
+}
+
+export function getAccessToken(): string | null {
+  if (typeof window === "undefined") return null
+  return window.localStorage.getItem(ACCESS_TOKEN_KEY) ?? window.sessionStorage.getItem(ACCESS_TOKEN_KEY)
+}
+
+export function getStoredUser(): AuthUser | null {
+  if (typeof window === "undefined") return null
+  const raw = window.localStorage.getItem(USER_KEY) ?? window.sessionStorage.getItem(USER_KEY)
+  if (!raw) return null
+  try {
+    return JSON.parse(raw) as AuthUser
+  } catch {
+    return null
+  }
+}
+
+export function clearSession() {
+  for (const key of [ACCESS_TOKEN_KEY, REFRESH_TOKEN_KEY, USER_KEY]) {
+    window.localStorage.removeItem(key)
+    window.sessionStorage.removeItem(key)
+  }
+}