refactor(auth): use icon library for provider icons

src/app/auth/components/auth-panel.tsx

@@ -3,6 +3,7 @@
  * 认证面板：顶层 switch（Facebook / Email）+ 悬浮返回按钮
  */
 import { useRouter } from "next/navigation";
+import { ChevronLeft } from "lucide-react";
 
 import { useAuthDispatch, useAuthState } from "@/stores/auth/auth-context";
 
@@ -36,19 +37,7 @@ export function AuthPanel() {
         onClick={handleBack}
         aria-label="Back"
       >
-        <svg
-          width="20"
-          height="20"
-          viewBox="0 0 24 24"
-          fill="none"
-          stroke="currentColor"
-          strokeWidth="2.5"
-          strokeLinecap="round"
-          strokeLinejoin="round"
-          aria-hidden="true"
-        >
-          <path d="M15 18l-6-6 6-6" />
-        </svg>
+        <ChevronLeft size={20} strokeWidth={2.5} aria-hidden="true" />
       </button>
 
       {state.authPanelMode === "facebook" ? (