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.module.css

@@ -0,0 +1,23 @@
+/* 原始 Dart: lib/ui/auth/auth_screen.dart
+ *
+ * 视觉规格（与 Dart 对齐）：
+ *   - 全屏 `bg_login.png` 背景（cover）
+ *   - 500px 内容列由 MobileShell 约束
+ *   - 内容列居中
+ */
+.fullBleed {
+  position: relative;
+  flex: 1 1 auto;
+  display: flex;
+  flex-direction: column;
+  min-height: 100dvh;
+  background: url("/images/auth/bg-login.png") center / cover no-repeat;
+}
+
+.content {
+  flex: 1 1 auto;
+  display: flex;
+  flex-direction: column;
+  justify-content: center;
+  padding: var(--spacing-lg);
+}