refactor(ui): extract auth and avatar components

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

@@ -3,10 +3,10 @@
  * 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";
 
+import { AuthBackButton } from "./auth-back-button";
 import { AuthEmailPanel } from "./auth-email-panel";
 import { AuthFacebookPanel } from "./auth-facebook-panel";
 import styles from "./auth-panel.module.css";
@@ -29,14 +29,7 @@ export function AuthPanel() {
 
   return (
     <div className={styles.panelShell}>
-      <button
-        type="button"
-        className={styles.backButton}
-        onClick={handleBack}
-        aria-label="Back"
-      >
-        <ChevronLeft size={20} strokeWidth={2.5} aria-hidden="true" />
-      </button>
+      <AuthBackButton onClick={handleBack} />
 
       {state.authPanelMode === "facebook" ? (
         <AuthFacebookPanel />