feat(auth): redesign auth UI to match Dart original

- Full-bleed bg_login.png background outside 500px MobileShell
- Facebook landing: logo + Facebook pill button + "Other Sign In Options" link
- Replace centered Dialog with true bottom sheet (drag handle, top radius 28px)
- Email panel: internal login/register toggle, no separate route
- Pink gradient primary button (#f96ADE → #f657A0)
- Pill input fields (46px, white, radius 24, pink 10% shadow)
- Pink circular checkbox + bold Privacy/Terms rich text (visual only, non-blocking)
- Floating 40x40 white back button (top:20/left:20)
- Wire AuthPanelModeChanged dispatch for panel switching
- New design tokens (--color-auth-*, --auth-*, --radius-bottom-sheet)
- New generic BottomSheet component
- Copy ic-logo-login.png (+@2x, +@3x) from Dart assets

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

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

@@ -1,30 +1,74 @@
 "use client";
 /**
- * 认证面板：顶层 switch（Facebook / Email）
+ * 认证面板：顶层 switch（Facebook / Email）+ 悬浮返回按钮
  *
  * 原始 Dart: lib/ui/auth/widgets/auth_panel.dart
+ *
+ * 行为对齐：
+ *   - 状态机：`authPanelMode`（"facebook" | "email"）由 XState 管理
+ *   - 切换派发 `AuthPanelModeChanged`
+ *   - 悬浮返回按钮（40×40 白圆，top:20, left:20）：
+ *     - facebook 模式：`router.back()`
+ *     - email 模式：派发 `AuthPanelModeChanged(facebook)`
+ *   - Email 面板的弹层 Facebook 按钮同样派发 `AuthPanelModeChanged(facebook)`
  */
-import { useAuthState } from "@/stores/auth/auth-context";
+import { useRouter } from "next/navigation";
+
+import { useAuthDispatch, useAuthState } from "@/stores/auth/auth-context";
 
-import { AuthFacebookPanel } from "./auth-facebook-panel";
 import { AuthEmailPanel } from "./auth-email-panel";
+import { AuthFacebookPanel } from "./auth-facebook-panel";
 import styles from "./auth-panel.module.css";
 
 export function AuthPanel() {
   const state = useAuthState();
+  const dispatch = useAuthDispatch();
+  const router = useRouter();
+
+  const switchToEmail = () =>
+    dispatch({ type: "AuthPanelModeChanged", mode: "email" });
+  const switchToFacebook = () =>
+    dispatch({ type: "AuthPanelModeChanged", mode: "facebook" });
+
+  const handleBack = () => {
+    if (state.authPanelMode === "email") {
+      switchToFacebook();
+    } else {
+      router.back();
+    }
+  };
+
   return (
-    <div className={styles.panel}>
+    <div className={styles.panelShell}>
+      <button
+        type="button"
+        className={styles.backButton}
+        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>
+      </button>
+
       {state.authPanelMode === "facebook" ? (
-        <AuthFacebookPanel
-          onSwitchToEmail={() => {
-            // 切换模式由外部 dispatch 触发；这里不直接处理
-            // 因为 auth-panel 受 `authPanelMode` 控制，调用方需要 dispatch `AuthPanelModeChanged`
-            // 但我们目前 AuthFacebookPanel 内部 click "Continue with Email" 直接调用 onSwitchToEmail
-            // 实际上本组件直接渲染对应 panel
+        <AuthFacebookPanel onSwitchToEmail={switchToEmail} />
+      ) : (
+        <AuthEmailPanel
+          onSwitchToFacebook={() => {
+            switchToFacebook();
           }}
         />
-      ) : (
-        <AuthEmailPanel onBack={() => undefined} />
       )}
     </div>
   );