feat(back-button): add dark variant and update tests for BackButton component

src/app/chat/components/chat-header.tsx

@@ -8,6 +8,7 @@
 import type { ReactNode } from "react";
 import { Lock, Menu } from "lucide-react";
 
+import { BackButton } from "@/app/_components";
 import { ROUTES } from "@/router/routes";
 import { useAppNavigator } from "@/router/use-app-navigator";
 
@@ -19,9 +20,9 @@ export interface ChatHeaderProps {
 export function ChatHeader({ isGuest, offerBanner }: ChatHeaderProps) {
   const navigator = useAppNavigator();
 
-  if (isGuest) {
-    return (
-      <header className="flex shrink-0 flex-col items-stretch bg-transparent p-0">
+  return (
+    <header className="flex shrink-0 flex-col items-stretch bg-transparent p-0">
+      {isGuest ? (
         <button
           type="button"
           className="flex w-full cursor-pointer items-center justify-center gap-(--spacing-sm,8px) border-0 bg-(--color-accent,#f84d96) px-(--spacing-md,12px) py-(--spacing-sm,8px) text-center text-(length:--responsive-caption,var(--font-size-sm,12px)) font-medium text-white"
@@ -35,29 +36,28 @@ export function ChatHeader({ isGuest, offerBanner }: ChatHeaderProps) {
           />
           <span>Sign up to unlock more features</span>
         </button>
-      </header>
-    );
-  }
+      ) : (
+        offerBanner
+      )}
 
-  return (
-    <header className="flex shrink-0 flex-col items-stretch bg-transparent p-0">
-      {offerBanner}
-
-      <div className="ml-(--spacing-md,12px) flex items-center justify-between px-(--spacing-md,12px) py-(--spacing-sm,8px)">
-        {/* 菜单按钮（点击 → push 到 /sidebar，与 Dart MenuButton 一致） */}
-        <button
-          type="button"
-          className="flex cursor-pointer items-center justify-center rounded-(--radius-full,999px) border-0 bg-[rgba(13,11,20,0.7)] p-(--spacing-sm,8px) text-(--color-text-primary,#fff)"
-          onClick={() => navigator.push(ROUTES.sidebar)}
-          aria-label="Menu"
-        >
-          <Menu
-            className="size-(--icon-size-xl,32px)"
-            size={24}
-            aria-hidden="true"
+      {!isGuest ? (
+        <div className="flex items-center justify-between px-(--spacing-md,12px) py-(--spacing-sm,8px)">
+          <BackButton
+            href={ROUTES.splash}
+            variant="dark"
+            ariaLabel="Back to home"
           />
-        </button>
-      </div>
+
+          <button
+            type="button"
+            className="flex size-(--responsive-icon-button-size,42px) cursor-pointer items-center justify-center rounded-(--radius-full,999px) border border-[rgba(255,255,255,0.12)] bg-[rgba(13,11,20,0.7)] text-(--color-text-primary,#fff) shadow-[0_10px_24px_rgba(0,0,0,0.2)] backdrop-blur-md transition-[background,transform] duration-150 hover:bg-[rgba(28,24,39,0.82)] active:scale-96 focus-visible:outline-2 focus-visible:outline-offset-3 focus-visible:outline-[#f657a0]"
+            onClick={() => navigator.push(ROUTES.sidebar)}
+            aria-label="Menu"
+          >
+            <Menu size={24} aria-hidden="true" />
+          </button>
+        </div>
+      ) : null}
     </header>
   );
 }