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/dimensions.css

@@ -32,4 +32,31 @@
   /* 聊天媒体 */
   --chat-media-max-width: 240px;
   --chat-media-max-height: 240px;
+
+  /* ==================== Auth 页尺寸（与 Dart AppDimensions / Spacing 对齐） ==================== */
+  /* 输入框 / 主按钮高度（46px = Dart AppDimensions.height46） */
+  --auth-field-height: 46px;
+
+  /* 弹层内第三方登录按钮高度（40px） */
+  --auth-social-button-height: 40px;
+
+  /* 主按钮固定宽度（Dart AppDimensions.buttonWidth = 334） */
+  --auth-primary-button-width: 334px;
+
+  /* Logo 高度（120px） */
+  --auth-logo-height: 120px;
+
+  /* Legal 圆形复选框 */
+  --auth-legal-checkbox-size: 16px;
+  --auth-legal-checkbox-inner-size: 10px;
+
+  /* 悬浮返回按钮（40×40 圆形） */
+  --auth-back-button-size: 40px;
+
+  /* 底部弹层圆角（28px = Dart AppRadius.radius28） */
+  --auth-bottom-sheet-radius: 28px;
+
+  /* 弹层 drag handle（40×4 胶囊） */
+  --auth-drag-handle-width: 40px;
+  --auth-drag-handle-height: 4px;
 }