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/_components/core/bottom-sheet.module.css

@@ -0,0 +1,34 @@
+/**
+ * 底部弹层样式
+ *
+ * 原始 Dart: showModalBottomSheet（透明背景，底部白卡 + 顶部圆角 radius28）
+ */
+.scrim {
+  position: fixed;
+  inset: 0;
+  z-index: 50;
+  display: flex;
+  align-items: flex-end;
+  justify-content: center;
+}
+
+.panel {
+  position: relative;
+  width: 100%;
+  max-width: 500px;
+  background: var(--color-auth-surface);
+  color: var(--color-auth-text-primary);
+  border-radius: var(--radius-bottom-sheet) var(--radius-bottom-sheet) 0 0;
+  box-shadow: 0 -8px 24px rgba(0, 0, 0, 0.18);
+  padding-bottom: env(safe-area-inset-bottom);
+  animation: slideUp 0.18s ease-out;
+}
+
+@keyframes slideUp {
+  from {
+    transform: translateY(100%);
+  }
+  to {
+    transform: translateY(0);
+  }
+}

src/app/_components/core/bottom-sheet.tsx

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+"use client";
+/**
+ * 通用底部弹层（Bottom Sheet）
+ *
+ * 原始 Dart: 散落在各处的 `showModalBottomSheet`（auth_other_options_dialog 等）
+ *
+ * 设计目标：
+ *   - portal 渲染到 body
+ *   - 固定底部、最大宽度 500px、顶部圆角 `--radius-bottom-sheet`
+ *   - ESC 关闭 / 点击 scrim 关闭 / 打开时锁定 body 滚动
+ *   - 简单 slide-up 动画（与原 Dart 行为接近）
+ *   - 调用方只关心内容布局
+ */
+import { type ReactNode, useEffect } from "react";
+import { createPortal } from "react-dom";
+
+import styles from "./bottom-sheet.module.css";
+
+export interface BottomSheetProps {
+  open: boolean;
+  onClose: () => void;
+  children: ReactNode;
+  /** 蒙层透明度（0-1）。默认 0.45。 */
+  scrimOpacity?: number;
+  /** 禁用点击外部 / ESC 关闭。 */
+  persistent?: boolean;
+  /** ARIA 标签。 */
+  ariaLabel?: string;
+}
+
+export function BottomSheet({
+  open,
+  onClose,
+  children,
+  scrimOpacity = 0.45,
+  persistent = false,
+  ariaLabel,
+}: BottomSheetProps) {
+  useEffect(() => {
+    if (!open || typeof document === "undefined") return;
+    const onKey = (e: KeyboardEvent) => {
+      if (e.key === "Escape" && !persistent) onClose();
+    };
+    document.addEventListener("keydown", onKey);
+    const prevOverflow = document.body.style.overflow;
+    document.body.style.overflow = "hidden";
+    return () => {
+      document.removeEventListener("keydown", onKey);
+      document.body.style.overflow = prevOverflow;
+    };
+  }, [open, onClose, persistent]);
+
+  if (!open || typeof document === "undefined") return null;
+
+  return createPortal(
+    <div
+      className={styles.scrim}
+      style={{ background: `rgba(0, 0, 0, ${scrimOpacity})` }}
+      onClick={(e) => {
+        if (!persistent && e.target === e.currentTarget) onClose();
+      }}
+    >
+      <div
+        role="dialog"
+        aria-modal="true"
+        aria-label={ariaLabel}
+        className={styles.panel}
+        onClick={(e) => e.stopPropagation()}
+      >
+        {children}
+      </div>
+    </div>,
+    document.body,
+  );
+}