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/tokens/colors.css

@@ -53,4 +53,34 @@
 
   /* 半透明 / 模糊 */
   --color-blur-background: rgba(13, 11, 20, 0.85);
+
+  /* ==================== Auth 页专用（与 Dart auth_screen.dart 对齐） ==================== */
+  /* 浅色表面（Auth 页内容卡片 / 弹层背景） */
+  --color-auth-surface: #ffffff;
+
+  /* 弹层内第三方登录按钮边框 */
+  --color-auth-border: #d5d7db;
+
+  /* Auth 页输入框占位符 */
+  --color-auth-input-placeholder: #999999;
+
+  /* Auth 页文字主色（深色，区别于全局白字） */
+  --color-auth-text-primary: #333333;
+  --color-auth-text-secondary: #9e9e9e;
+
+  /* Auth 输入框阴影（accent #f759a8 10% 透明） */
+  --color-auth-input-shadow: rgba(247, 89, 168, 0.10);
+
+  /* Auth 主按钮阴影（#d00c41 20% 透明） */
+  --color-auth-primary-button-shadow: rgba(208, 12, 65, 0.20);
+
+  /* Auth 主按钮渐变（与 Dart primaryGradient 对齐） */
+  --color-auth-primary-gradient-start: #f96ade;
+  --color-auth-primary-gradient-end: #f657a0;
+
+  /* Legal 复选框选中点 */
+  --color-auth-legal-check: #f657a0;
+
+  /* 底部弹层 drag handle */
+  --color-auth-drag-handle: #3a3a3a;
 }