feat(app): improve payment and paywall UX

src/app/_components/core/auth-back-button.module.css

@@ -1,22 +0,0 @@
-.button {
-  display: inline-flex;
-  align-items: center;
-  justify-content: center;
-  width: 40px;
-  height: 40px;
-  border: none;
-  border-radius: var(--radius-full);
-  background: transparent;
-  color: var(--color-text-primary);
-  cursor: pointer;
-  transition: background 0.15s ease;
-}
-
-.button:hover {
-  background: rgba(255, 255, 255, 0.08);
-}
-
-.button:focus-visible {
-  outline: var(--border-medium) solid var(--color-accent);
-  outline-offset: 2px;
-}

src/app/_components/core/auth-back-button.tsx

@@ -1,59 +0,0 @@
-"use client";
-/**
- * 通用返回按钮
- *
- * 原始 Dart: lib/ui/auth/widgets/back_button.dart（被 auth/chat/sidebar 共用）。
- */
-import { useRouter } from "next/navigation";
-import { type MouseEvent } from "react";
-
-import styles from "./auth-back-button.module.css";
-
-export interface AuthBackButtonProps {
-  /** 自定义跳转目标（默认 history.back）。 */
-  href?: string;
-  /** 自定义点击处理（覆盖默认跳转）。 */
-  onClick?: (e: MouseEvent<HTMLButtonElement>) => void;
-  className?: string;
-  ariaLabel?: string;
-}
-
-export function AuthBackButton({
-  href,
-  onClick,
-  className,
-  ariaLabel = "Back",
-}: AuthBackButtonProps) {
-  const router = useRouter();
-  return (
-    <button
-      type="button"
-      aria-label={ariaLabel}
-      className={[styles.button, className].filter(Boolean).join(" ")}
-      onClick={(e) => {
-        if (onClick) {
-          onClick(e);
-          return;
-        }
-        if (href) router.push(href);
-        else router.back();
-      }}
-    >
-      <svg
-        width="20"
-        height="20"
-        viewBox="0 0 24 24"
-        fill="none"
-        aria-hidden="true"
-      >
-        <path
-          d="M15 18l-6-6 6-6"
-          stroke="currentColor"
-          strokeWidth="2"
-          strokeLinecap="round"
-          strokeLinejoin="round"
-        />
-      </svg>
-    </button>
-  );
-}

src/app/_components/core/index.ts

@@ -2,7 +2,6 @@
  * @file Automatically generated by barrelsby.
  */
 
-export * from "./auth-back-button";
 export * from "./bottom-sheet";
 export * from "./checkbox";
 export * from "./dialog";