feat: 迁移珊瑚橙主题体系并新增认证基础组件（ADR-005）

- app_theme.dart 重写为语义 token（AppColors/AppRadius），落地 primaryStrong/error 等 AA 对比度色
- 五个页面与公共组件的旧靛蓝硬编码色值全部替换为 token，仅换色不动布局
- 新增 BrandMark/AppTextField/PrimaryButton/InlineErrorBanner/AuthScaffold 及 6 个 widget 测试
- 门禁：dart format（0 changed）/ flutter analyze（0 issues）/ flutter test（7 passed）

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

lib/features/home/home_page.dart

@@ -314,15 +314,19 @@ class _PetGreetingCard extends StatelessWidget {
         gradient: const LinearGradient(
           begin: Alignment.topLeft,
           end: Alignment.bottomRight,
-          colors: [Color(0xFFECFDF5), Color(0xFFFFF7ED)],
+          colors: [AppColors.surfaceTint, AppColors.canvas],
         ),
       ),
       child: Stack(
         children: [
-          const Positioned(
+          Positioned(
             left: 160,
             top: 15,
-            child: Icon(Icons.pets, size: 28, color: Color(0x224F46E5)),
+            child: Icon(
+              Icons.pets,
+              size: 28,
+              color: AppColors.primary.withAlpha(34),
+            ),
           ),
           Positioned(
             right: -5,
@@ -355,7 +359,7 @@ class _PetGreetingCard extends StatelessWidget {
                   maxLines: 3,
                   overflow: TextOverflow.ellipsis,
                   style: const TextStyle(
-                    color: Color(0xFF475569),
+                    color: AppColors.primaryDark,
                     fontSize: 12,
                     height: 1.45,
                     fontWeight: FontWeight.w600,
@@ -497,7 +501,7 @@ class _WeatherDetailsSheet extends StatelessWidget {
             width: double.infinity,
             padding: const EdgeInsets.all(16),
             decoration: BoxDecoration(
-              color: const Color(0xFFEEF2FF),
+              color: AppColors.surfaceTint,
               borderRadius: BorderRadius.circular(18),
             ),
             child: Row(
@@ -527,10 +531,10 @@ IconData _weatherIcon(WeatherCondition condition) {
 
 Color _weatherColor(WeatherCondition condition) {
   return switch (condition) {
-    WeatherCondition.sunny => const Color(0xFFF59E0B),
-    WeatherCondition.cloudy => const Color(0xFF64748B),
+    WeatherCondition.sunny => AppColors.warning,
+    WeatherCondition.cloudy => AppColors.muted,
     WeatherCondition.rainy => const Color(0xFF0284C7),
-    WeatherCondition.overcast => const Color(0xFF475569),
+    WeatherCondition.overcast => AppColors.ink,
     WeatherCondition.snow => const Color(0xFF0891B2),
   };
 }
@@ -577,9 +581,7 @@ class _StoryRow extends StatelessWidget {
                     padding: const EdgeInsets.all(2),
                     decoration: const BoxDecoration(
                       shape: BoxShape.circle,
-                      gradient: LinearGradient(
-                        colors: [AppColors.primary, Color(0xFF8B5CF6)],
-                      ),
+                      gradient: AppColors.brandGradient,
                     ),
                     child: index == 0
                         ? const CircleAvatar(
@@ -617,9 +619,7 @@ class _PromoCard extends StatelessWidget {
     return Container(
       padding: const EdgeInsets.all(20),
       decoration: BoxDecoration(
-        gradient: const LinearGradient(
-          colors: [AppColors.primary, Color(0xFF7C3AED)],
-        ),
+        gradient: AppColors.brandGradient,
         borderRadius: BorderRadius.circular(26),
       ),
       child: Row(
@@ -639,7 +639,7 @@ class _PromoCard extends StatelessWidget {
                 SizedBox(height: 5),
                 Text(
                   '遛狗 / 洗澡 / 寄养均可使用',
-                  style: TextStyle(color: Color(0xFFE0E7FF), fontSize: 12),
+                  style: TextStyle(color: Colors.white, fontSize: 12),
                 ),
               ],
             ),
@@ -647,7 +647,7 @@ class _PromoCard extends StatelessWidget {
           FilledButton(
             style: FilledButton.styleFrom(
               backgroundColor: Colors.white,
-              foregroundColor: AppColors.primary,
+              foregroundColor: AppColors.primaryStrong,
             ),
             onPressed: onTap,
             child: const Text('去使用'),
@@ -789,7 +789,7 @@ class _CategoryGrid extends StatelessWidget {
               mainAxisAlignment: MainAxisAlignment.center,
               children: [
                 CircleAvatar(
-                  backgroundColor: const Color(0xFFEEF2FF),
+                  backgroundColor: AppColors.surfaceTint,
                   child: Icon(item.$2, color: AppColors.primary),
                 ),
                 const SizedBox(height: 7),