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-screen.tsx

@@ -5,10 +5,11 @@
  * 原始 Dart: lib/ui/auth/auth_screen.dart
  *
  * 行为对齐：
- *  - 500px 移动端宽度（MobileShell）
- *  - 已登录用户：自动跳 /chat
- *  - 未登录：渲染 AuthPanel（含 Facebook/Email 两种面板）
- *  - 登录成功：通知 ChatBloc + UserBloc，然后 router.replace(/chat)
+ *   - 500px 移动端宽度（MobileShell）
+ *   - 全屏 `bg_login.png` 背景（cover 模式，扩展到 viewport）
+ *   - 已登录用户：自动跳 /chat
+ *   - 未登录：渲染 AuthPanel（含 Facebook/Email 两种面板）
+ *   - 登录成功：通知 ChatBloc + UserBloc，然后 router.replace(/chat)
  */
 import { useEffect, useRef } from "react";
 import { useRouter } from "next/navigation";
@@ -20,14 +21,9 @@ import { ROUTES } from "@/router/routes";
 import { MobileShell } from "@/app/_components/core/mobile-shell";
 import { AuthPanel } from "@/app/auth/components/auth-panel";
 
-export interface AuthScreenProps {
-  /** 自定义背景（默认紫粉渐变）。 */
-  background?: string;
-}
+import styles from "./auth-screen.module.css";
 
-export function AuthScreen({
-  background = "var(--color-sidebar-background)",
-}: AuthScreenProps) {
+export function AuthScreen() {
   // ===== 鉴权路由跳转已由 src/router/proxy.ts 集中处理 =====
   // 本组件只负责：
   //   1. 渲染登录面板
@@ -50,18 +46,12 @@ export function AuthScreen({
   }, [state.isSuccess, chatDispatch, userDispatch, router]);
 
   return (
-    <MobileShell background={background}>
-      <div
-        style={{
-          flex: 1,
-          display: "flex",
-          flexDirection: "column",
-          justifyContent: "center",
-          padding: "var(--spacing-lg)",
-        }}
-      >
-        <AuthPanel />
-      </div>
-    </MobileShell>
+    <div className={styles.fullBleed}>
+      <MobileShell>
+        <div className={styles.content}>
+          <AuthPanel />
+        </div>
+      </MobileShell>
+    </div>
   );
 }