fix(client): 登录页 tab 滑块塌陷 + 滚动弹性(design-distill 校准)

- 胶囊 tab 活动滑块高度塌成 0(FractionallySizedBox 缺 heightFactor)→ 活动态
  不可见、和背景糊一起。补 heightFactor:1.0,滑块正常填满。
- 列表无弹性回弹:SingleChildScrollView 改 ClampingScrollPhysics,大窗口放得下
  就不滚(无上下拖动/滚动条),仅窗口过矮才滚。
- 加 auth_redesign golden(900x700 明/暗),按 design-distill 截图 diff 校准:
  与原型 design/preview/auth_redesign.html 对比,结构/位置/尺寸零偏移
  (残余差异为 headless 测试缺 CJK 字体的豆腐块,真 app 字体正常)。

flutter analyze 0 error;测试通过。

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

client/lib/widgets/auth_screen.dart

@@ -154,6 +154,9 @@ class _AuthScreenState extends ConsumerState<AuthScreen>
         SafeArea(
           child: Center(
             child: SingleChildScrollView(
+              // 无弹性回弹:内容放得下就不滚(大窗口不出现上下拖动/滚动条),
+              // 仅窗口太矮放不下时才滚动。
+              physics: const ClampingScrollPhysics(),
               padding: const EdgeInsets.symmetric(horizontal: 24, vertical: 32),
               child: ConstrainedBox(
                 constraints: const BoxConstraints(maxWidth: 420),
@@ -277,6 +280,7 @@ class _AuthScreenState extends ConsumerState<AuthScreen>
             alignment: reg ? Alignment.centerRight : Alignment.centerLeft,
             child: FractionallySizedBox(
               widthFactor: 0.5,
+              heightFactor: 1.0,
               child: DecoratedBox(
                 decoration: BoxDecoration(
                   color: c.accent,

client/test/golden/auth_redesign_golden_test.dart

@@ -0,0 +1,52 @@
+// auth_redesign_golden_test.dart — 登录页 Flutter 渲染图(供 design-distill 截图 diff)
+// 900x700 @1x,与原型截图同 viewport;明/暗各一张。
+import 'package:flutter/material.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:pangolin_vpn/l10n/strings_zh.dart';
+import 'package:pangolin_vpn/pangolin_theme.dart';
+import 'package:pangolin_vpn/services/token_store.dart';
+import 'package:pangolin_vpn/state/auth_provider.dart';
+import 'package:pangolin_vpn/widgets/auth_screen.dart';
+
+class _NullTokenStore implements TokenStore {
+  const _NullTokenStore();
+  @override
+  Future<void> saveTokens({required String access, required String refresh}) async {}
+  @override
+  Future<String?> loadAccessToken() async => null;
+  @override
+  Future<String?> loadRefreshToken() async => null;
+  @override
+  Future<void> clear() async {}
+  @override
+  Future<void> markOnboarded() async {}
+  @override
+  Future<bool> isOnboarded() async => true;
+  @override
+  Future<void> saveLastEmail(String email) async {}
+  @override
+  Future<String?> loadLastEmail() async => null;
+}
+
+Future<void> _shoot(WidgetTester tester, ThemeData theme, String name) async {
+  tester.view.physicalSize = const Size(900, 700);
+  tester.view.devicePixelRatio = 1.0;
+  addTearDown(tester.view.resetPhysicalSize);
+  addTearDown(tester.view.resetDevicePixelRatio);
+  await tester.pumpWidget(ProviderScope(
+    overrides: [tokenStoreProvider.overrideWithValue(const _NullTokenStore())],
+    child: MaterialApp(
+      debugShowCheckedModeBanner: false,
+      theme: theme,
+      home: AuthScreen(t: const StringsZh(), onDone: () {}),
+    ),
+  ));
+  await tester.pumpAndSettle();
+  await expectLater(find.byType(AuthScreen), matchesGoldenFile('goldens/$name.png'));
+}
+
+void main() {
+  testWidgets('auth login light', (t) => _shoot(t, PangolinTheme.light, 'auth_login_light'));
+  testWidgets('auth login dark', (t) => _shoot(t, PangolinTheme.dark, 'auth_login_dark'));
+}