fix: 修复推荐码输入框颜色问题

- 改为白色背景(alpha: 0.9)，更不透明
- 文字颜色改为 black87，清晰可见
- 边框改为绿色，更突出
- 提示文字改为灰色，易于区分

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

frontend/mobile-app/lib/features/auth/presentation/pages/guide_page.dart

@@ -586,11 +586,11 @@ class _WelcomePageContentState extends ConsumerState<_WelcomePageContent> {
         Container(
           padding: EdgeInsets.symmetric(vertical: 12.h, horizontal: 16.w),
           decoration: BoxDecoration(
-            color: Colors.white.withValues(alpha: 0.15),
+            color: Colors.white.withValues(alpha: 0.9), // 改为更不透明的背景
             borderRadius: BorderRadius.circular(12.r),
             border: Border.all(
-              color: Colors.white.withValues(alpha: 0.3),
-              width: 1,
+              color: const Color(0xFF4CAF50), // 绿色边框
+              width: 1.5,
             ),
           ),
           child: Row(
@@ -602,7 +602,7 @@ class _WelcomePageContentState extends ConsumerState<_WelcomePageContent> {
                     hintText: '请输入推荐码或点击右侧扫码',
                     hintStyle: TextStyle(
                       fontSize: 15.sp,
-                      color: Colors.white.withValues(alpha: 0.5),
+                      color: Colors.grey[500], // 改为灰色提示文字
                     ),
                     border: InputBorder.none,
                     isDense: true,
@@ -610,7 +610,8 @@ class _WelcomePageContentState extends ConsumerState<_WelcomePageContent> {
                   ),
                   style: TextStyle(
                     fontSize: 16.sp,
-                    color: Colors.white,
+                    color: Colors.black87, // 改为深色文字，清晰可见
+                    fontWeight: FontWeight.w500,
                   ),
                 ),
               ),