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-email-panel.tsx

@@ -1,47 +1,117 @@
 "use client";
 /**
- * Email 面板
+ * Email 面板（login/register 内部切换）
  *
  * 原始 Dart: lib/ui/auth/widgets/auth_email_panel.dart
  *
- * 行为：
- *  - 内部 `authMode`（login/register）切换显示 Login / Register 表单
- *  - 提交派发 `AuthEmailLoginSubmitted` 或 `AuthEmailRegisterSubmitted`
+ * 视觉规格（与 Dart 对齐）：
+ *   - login 模式：顶部 24px spacer + logo（120h） + 24px gap
+ *   - register 模式：48px spacer（两个 24px，无 logo）
+ *   - 表单（EmailLoginForm / EmailRegisterForm）
+ *   - 12px gap + 切换链接（"Don't have an account? Sign up" / "Already have an account? Log in"）
+ *   - 16px gap + "Other Sign In Options" 链接（打开底部弹层）
+ *   - Spacer + AuthLegalText
+ *   - 顶部 "← Back" 按钮由父级 AuthPanel 渲染（悬浮 40×40 圆形）
  */
 import { useState } from "react";
 
 import { useAuthState } from "@/stores/auth/auth-context";
 
+import { AuthErrorMessage } from "./auth-error-message";
+import { AuthLegalText } from "./auth-legal-text";
+import { AuthOtherOptionsDialog } from "./auth-other-options-dialog";
 import { EmailLoginForm } from "./email-login-form";
 import { EmailRegisterForm } from "./email-register-form";
 import styles from "./auth-email-panel.module.css";
 
 export interface AuthEmailPanelProps {
-  onBack: () => void;
+  /** "Other Sign-in Options" 弹层中 Facebook 按钮回调（派发回 facebook 模式） */
+  onSwitchToFacebook: () => void;
+  /** "Other Sign-in Options" 弹层中 Google 按钮回调（可空；空时不渲染 Google 行） */
+  onGoogle?: () => void;
+  /** 弹层 Google 自定义子节点 */
+  googleSlot?: React.ReactNode;
 }
 
-export function AuthEmailPanel({ onBack }: AuthEmailPanelProps) {
+export function AuthEmailPanel({
+  onSwitchToFacebook,
+  onGoogle,
+  googleSlot,
+}: AuthEmailPanelProps) {
   const state = useAuthState();
   const [mode, setMode] = useState<"login" | "register">("login");
+  const [showOptions, setShowOptions] = useState(false);
 
   return (
     <div className={styles.panel}>
-      <button type="button" className={styles.backButton} onClick={onBack}>
-        ← Back
-      </button>
+      <div className={styles.topSection}>
+        {mode === "login" ? (
+          <>
+            <div className={styles.spacer24} />
+            <img
+              src="/images/auth/ic-logo-login.png"
+              alt="Cozsweet"
+              className={styles.logo}
+            />
+          </>
+        ) : (
+          <>
+            <div className={styles.spacer24} />
+            <div className={styles.spacer24} />
+          </>
+        )}
+        <div className={styles.spacer24} />
+      </div>
+
       {mode === "login" ? (
         <EmailLoginForm
-          onSwitchToRegister={() => setMode("register")}
           globalError={state.errorMessage}
           isLoading={state.isLoading}
         />
       ) : (
         <EmailRegisterForm
-          onSwitchToLogin={() => setMode("login")}
           globalError={state.errorMessage}
           isLoading={state.isLoading}
         />
       )}
+
+      <AuthErrorMessage message={state.errorMessage} />
+
+      <button
+        type="button"
+        className={styles.toggleButton}
+        onClick={() => setMode((m) => (m === "login" ? "register" : "login"))}
+      >
+        {mode === "login"
+          ? "Don't have an account? Sign up"
+          : "Already have an account? Log in"}
+      </button>
+
+      <button
+        type="button"
+        className={styles.linkButton}
+        onClick={() => setShowOptions(true)}
+      >
+        Other Sign In Options
+      </button>
+
+      <div className={styles.spacer} />
+      <AuthLegalText />
+
+      <AuthOtherOptionsDialog
+        open={showOptions}
+        onClose={() => setShowOptions(false)}
+        mode="email"
+        onFacebook={onSwitchToFacebook}
+        onGoogle={() => {
+          setShowOptions(false);
+          onGoogle?.();
+        }}
+        onEmail={() => {
+          // 在 email 模式下不再显示 Email 入口
+        }}
+        googleSlot={googleSlot}
+      />
     </div>
   );
 }