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

src/app/_components/__tests__/tailwind-components.test.tsx

@@ -13,6 +13,9 @@ describe("shared Tailwind components", () => {
     const actionHtml = renderToStaticMarkup(
       <BackButton onClick={() => undefined} variant="ghost" />,
     );
+    const darkHtml = renderToStaticMarkup(
+      <BackButton href="/splash" variant="dark" ariaLabel="Back to home" />,
+    );
 
     expect(linkHtml).toContain('href="/chat"');
     expect(linkHtml).toContain('aria-label="Back to chat"');
@@ -22,6 +25,10 @@ describe("shared Tailwind components", () => {
     expect(actionHtml).toContain("<button");
     expect(actionHtml).toContain("size-8");
     expect(actionHtml).toContain("hover:opacity-70");
+    expect(darkHtml).toContain('href="/splash"');
+    expect(darkHtml).toContain('aria-label="Back to home"');
+    expect(darkHtml).toContain("bg-[rgba(13,11,20,0.7)]");
+    expect(darkHtml).toContain("text-white");
   });
 
   it("renders CharacterAvatar with dynamic sizing and image utilities", () => {

src/app/_components/back-button.tsx

@@ -7,7 +7,7 @@ interface BackButtonBaseProps {
   className?: string;
   ariaLabel?: string;
   iconSize?: number;
-  variant?: "floating" | "soft" | "ghost";
+  variant?: "floating" | "soft" | "ghost" | "dark";
 }
 
 type BackButtonLinkProps = BackButtonBaseProps & {
@@ -32,6 +32,8 @@ const VARIANT_CLASS_NAMES = {
     "size-(--responsive-icon-button-size,42px) border border-[rgba(25,19,22,0.08)] bg-[rgba(255,255,255,0.82)] text-[#21191d] shadow-[0_10px_22px_rgba(50,30,40,0.08)] hover:-translate-y-px hover:bg-white hover:shadow-[0_14px_26px_rgba(50,30,40,0.12)] active:translate-y-px",
   ghost:
     "size-8 bg-transparent text-black hover:opacity-70 active:scale-96",
+  dark:
+    "size-(--responsive-icon-button-size,42px) border border-[rgba(255,255,255,0.12)] bg-[rgba(13,11,20,0.7)] text-white shadow-[0_10px_24px_rgba(0,0,0,0.2)] backdrop-blur-md hover:bg-[rgba(28,24,39,0.82)] active:scale-96",
 } satisfies Record<NonNullable<BackButtonBaseProps["variant"]>, string>;
 
 export function BackButton({

src/app/chat/components/__tests__/tailwind-components.test.tsx

@@ -160,10 +160,15 @@ describe("chat Tailwind components", () => {
     expect(guestHtml).toContain('aria-label="Sign up to unlock more features"');
     expect(guestHtml).toContain("bg-(--color-accent,#f84d96)");
     expect(guestHtml).toContain("size-(--icon-size-sm,16px)");
+    expect(guestHtml).not.toContain('href="/splash"');
+    expect(guestHtml).not.toContain('aria-label="Back to home"');
+    expect(guestHtml).not.toContain('aria-label="Menu"');
     expect(memberHtml).toContain("Offer");
+    expect(memberHtml).toContain('href="/splash"');
+    expect(memberHtml).toContain('aria-label="Back to home"');
     expect(memberHtml).toContain('aria-label="Menu"');
     expect(memberHtml).toContain("bg-[rgba(13,11,20,0.7)]");
-    expect(memberHtml).toContain("size-(--icon-size-xl,32px)");
+    expect(memberHtml).toContain("size-(--responsive-icon-button-size,42px)");
   });
 
   it("renders ChatInputTextField with textarea utilities", () => {

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">
+      {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 (
+      {!isGuest ? (
-    <header className="flex shrink-0 flex-col items-stretch bg-transparent p-0">
+        <div className="flex items-center justify-between px-(--spacing-md,12px) py-(--spacing-sm,8px)">
-      {offerBanner}
+          <BackButton
+            href={ROUTES.splash}
+            variant="dark"
+            ariaLabel="Back to home"
+          />
 
-      <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)"
+            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
+            <Menu size={24} aria-hidden="true" />
-            className="size-(--icon-size-xl,32px)"
-            size={24}
-            aria-hidden="true"
-          />
           </button>
         </div>
+      ) : null}
     </header>
   );
 }