fix: 修复UI样式问题

- 首页：移除盈亏日历，福利中心卡片改为白色背景，查看按钮改为黄色
- 资产页面：余额卡片撑满宽度
- 底部导航栏：移除背景高亮，只保留图标和文字变化
- 行情页面：调整卡片文字大小和柱形图样式
- 交易页面：买入按钮白色文字，卖出按钮红色文字，优化输入框和百分比卡片样式，添加顶部间距
- 全局：移除渐变色，统一使用纯色背景

flutter_monisuo/lib/ui/pages/main/main_page.dart

@@ -142,15 +142,17 @@ class _BottomNavBar extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
-    final colorScheme = Theme.of(context).colorScheme;
+    final isDark = Theme.of(context).brightness == Brightness.dark;
+    // Light: #FFFFFF, Dark: #0F172A
+    final backgroundColor = isDark ? const Color(0xFF0F172A) : const Color(0xFFFFFFFF);
 
     return Container(
       decoration: BoxDecoration(
-        color: colorScheme.surfaceContainerLow,
+        color: backgroundColor,
         borderRadius: BorderRadius.vertical(top: Radius.circular(AppRadius.xxl + AppSpacing.sm)),
         border: Border(
           top: BorderSide(
-            color: colorScheme.outlineVariant.withValues(alpha: 0.15),
+            color: Theme.of(context).colorScheme.outlineVariant.withValues(alpha: 0.15),
           ),
         ),
       ),
@@ -193,15 +195,8 @@ class _NavItemWidget extends StatelessWidget {
     return GestureDetector(
       onTap: onTap,
       behavior: HitTestBehavior.opaque,
-      child: AnimatedContainer(
-        duration: const Duration(milliseconds: 300),
+      child: Padding(
         padding: EdgeInsets.symmetric(horizontal: AppSpacing.md, vertical: AppSpacing.sm),
-        decoration: isSelected
-            ? BoxDecoration(
-                color: colorScheme.primary.withValues(alpha: 0.1),
-                borderRadius: AppRadius.radiusMd,
-              )
-            : null,
         child: Column(
           mainAxisSize: MainAxisSize.min,
           children: [