feat(client): 登录屏离线提示条 token 化 + 整屏 golden ×三主题

服务器不可达提示条 FFF8E1/F57F17/5D4037 → warnBg/warn；
品牌 hero + 表单整屏 golden 入回归闸（覆盖联网首检避免 Timer 泄漏）。

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TSKEiHsvauyxYUW2itzUXX

client/lib/screens/auth/login_screen.dart

@@ -546,27 +546,27 @@ class _LoginScreenState extends ConsumerState<LoginScreen> {
                                 padding: const EdgeInsets.symmetric(
                                     horizontal: 12, vertical: 10),
                                 decoration: BoxDecoration(
-                                  color: const Color(0xFFFFF8E1),
+                                  color: context.tokens.warnBg,
                                   borderRadius: BorderRadius.circular(4),
                                   border: Border.all(
-                                      color: const Color(0xFFF57F17)
-                                          .withAlpha(120)),
+                                      color:
+                                          context.tokens.warn.withAlpha(120)),
                                 ),
-                                child: const Row(
+                                child: Row(
                                   children: [
                                     Icon(Icons.wifi_off,
-                                        color: Color(0xFFF57F17), size: 16),
-                                    SizedBox(width: 8),
+                                        color: context.tokens.warn, size: 16),
+                                    const SizedBox(width: 8),
                                     Expanded(
                                       child: Text(
                                         '服务器不可达，请检查网络连接',
                                         style: TextStyle(
-                                            color: Color(0xFF5D4037),
+                                            color: context.tokens.warn,
                                             fontSize: 13),
                                       ),
                                     ),
                                     NetworkRetryButton(
-                                        foreground: Color(0xFF5D4037)),
+                                        foreground: context.tokens.warn),
                                   ],
                                 ),
                               ),